This is player development.

Prospects rarely travel straight lines through their minor league careers. Almost all must endure struggle at some stage of their careers -- some at the beginning of their time in the minors, others at higher levels.

Some who are at one point overmatched will never recover, leaving promising careers to end in disappointment. Others will regain their footing and ultimately meet or exceed the once-enormous expectations generated by their evident talent.

Because they must inevitably encounter struggle and help steer minor leaguers past it, player development officials are conditioned to embrace adversity. Even as they lose sleep while thinking about what can be done to help a prospect correct his course, they point to struggles as a crucial phenomenon that will serve as a critical teaching tool in the eventual emergence of big leaguers.

“Nobody’s going to feel sorry for you. You’ve got to push through. You’ve got to be tough. To play in Boston someday, you’ve got to be thick-skinned. You have to really have thick skin by the time you get here,” Kevin Boles, the manager of the Red Sox’ Double-A Portland affiliate, said recently. “We’re trying to prepare players for one of the biggest markets in the world -- Boston, New York, Philadelphia, these are special markets. Our guys have to be hardened. When they get to the big leagues, they have to be ready to take the criticism, whether from the fan base, or from the media. We have to harden these guys up.

“The struggles, having them hardened from learning from a little bit of failure, that can only be beneficial. If they’ve never struggled and once they get to the big leagues they start struggling, now they’re not going to know how to handle it. If they struggle sometime in their minor league career, they’ve been there, OK, how did they handle it? How did they push through?”

Boles’ perspective on the matter is particularly interesting. In 2009, he managed Single-A Greenville before moving up the ladder in the Red Sox farm system, first to High-A Salem in 2010 and then to Double-A Portland in 2011. His progression thus occurred in lockstep over the last three seasons with a trio of Red Sox prospects who began the year in Portland but who followed divergent paths in 2011.

Entering the season, right-hander Stolmy Pimentel was ranked the sixth-best prospect in the Red Sox system by Baseball America based on a consistent track record of success dating to the time when he was signed for $25,000 as a 16-year-old out of the Dominican Republic. Second baseman Oscar Tejeda’s breakout year in 2010 at Salem earned him the No. 10 ranking among Sox prospects, one spot ahead of third baseman Will Middlebrooks.

Now, those rankings have been shaken up significantly.

By the middle of the year, Middlebrooks had become the top prospect in the Red Sox system, a player who combined significant power potential with an ability to hit for solid average while playing above-average defense at third base. He looks like a future middle-of-the-order corner infielder, someone who is positioning himself as the potential successor to Kevin Youkilis at the hot corner by 2013 or so.

Middlebrooks’ emergence has been impressive in its own right, but it is little short of astonishing given his stature in the system entering the year. Even after the Adrian Gonzalez trade sent three top minor leaguers packing to San Diego, he could not crack the top 10 lists of Sox prospects. The memories of his significant struggles both in his first professional season in 2008 with short-season Lowell and again in 2009 with Greenville tempered the enthusiasm generated by his strong 2010 season in Salem.

Based on their pre-2011 career arcs and tools, it was right-hander Pimentel (9-11 with a 4.06 ERA, 7.1 strikeouts per nine innings and 2.9 walks per nine innings in Salem at age 20) who appeared the closest and most certain to make a big league impact.

Tejeda, meanwhile, had enjoyed a breakout year after being moved from shortstop to second base, hitting .307 with a .344 OBP, a .455 slugging mark, a .799 OPS and 11 homers as a 20-year-old in Salem.

As for Middlebrooks, he had seen his 2010 season tail off after a scorching start, hitting .276 with a .331 OBP, a .439 slugging mark, a .770 OPS and 12 homers at age 21 in Salem. Given both his age and how his numbers at the end of the year stacked up against Tejeda’s, the idea that he was behind both of his two peers in Salem was difficult to argue.

In 2011, however, the three prospects moved in strikingly different directions. All three started the year in Double-A Portland; in the just-concluded minor league regular season, they all finished their years in different places.

Middlebrooks enjoyed a monster year. In his 96 games in Double-A, he hit .302 with a .345 OBP, a .520 slugging mark, a .865 OPS and 18 homers (plus an additional three homers in a four-game rehab stint in Lowell). He was leading the Eastern League with 80 RBI at the time of his promotion to Triple-A Pawtucket in late August.

While there was an adjustment to the more advanced pitching of the highest level of the minors, Middlebrooks hit two homers in his final three games of the minor league season, including a grand slam against highly regarded Yankees southpaw Manny Banuelos on Saturday. In 16 Triple-A games, he hit .161 with a .200 OBP, .268 slugging mark and .468 OPS. 

His year has been richly decorated. He participated in the All-Star Futures Game, was selected for the Double-A All-Star team, was named the best hitting prospect in the Eastern League by managers and scouts and will head out next month to play for an Arizona Fall League team that features Bryce Harper and Mike Trout, the top prospects in the minors.

“Each successive year, he’s been more and more consistent,” Sox farm director Mike Hazen said. “This year, he kind of put it all together for the first time.”

While Middlebrooks enjoyed tremendous success in Double-A, his aggressive approach at the plate represents one area for further development and refinement. He walked just 21 times while striking out 95 times in 397 Double-A plate appearances.

While he will need to do more to control his at-bats, especially in Triple-A and ultimately the majors against veteran pitchers who try to exploit his aggressiveness, the Sox insist that at this stage of his development, Middlebrooks’ approach is not an issue.

“He’s a guy who will take some risks at the plate because he’s looking to impact the baseball,” said Boles. “[Ryan] Kalish is an example of that. [Anthony] Rizzo is another example. He’s falling right in line with those types of players. … The guys who have a chance to be impact guys, they’re going to take risks.”

Tejeda, meanwhile, opened the year with tremendous promise, impressing Sox staffers in his first big league spring training. He hit .360 with a .407 OBP and 1.047 OPS in 14 games before being optioned to minor league camp.

But when the season began, Tejeda proved unable to carry those results forward. He spent the entire year in Portland, hitting .249 with a .297 OBP, a .339 slugging mark, a .636 OPS, five homers and 13 steals. Across the board, he produced below-average numbers in the Eastern League.

Whereas he consistently impacted the ball at Salem the previous year, he proved unable to do so in Portland. Still, the Sox suggest a broader view when it comes to the toolsy 21-year-old, who began the year as the third youngest position player in the league.

“He’s had a very solid year. The numbers aren’t what they were in Salem. But he’s 21 years old going into the season, and to see the improvements in how he’s trying to manage his ABs, the defense at second base is improving, this is a kid, there’s a lot of potential,” said Boles. “He’s 21 years old. Where would he be coming out in the draft? You’ve got to keep that perspective.”

The same can be said of Pimentel. The right-hander commanded plenty of attention this year in his first big league camp. Though 21, he arrived in camp with the physique and stuff of a big league pitcher.

He was running his fastball to the plate at 95 mph in Fort Myers, featured a swing-and-miss changeup that has always been his out pitch and showed an ability to spin a breaking ball, albeit with erratic results. It was not unreasonable to suggest that he could be a candidate for a September call-up.

Instead, his season has moved in the opposite direction. Pimentel went 0-9 with a 9.12 ERA in Portland, resulting in a July demotion back to Salem.

He got off to a poor start, allowing nine runs in 4 1/3 innings over his first two appearances. But in his final eight appearances for Salem, he went 5-2 with a 3.24 ERA, striking out 27 and walking 11.

“At the end of the year, I still think this may end up being Pimentel’s best development year that he ever has,” said Hazen. “He’s now making the adjustments we asked him to make, and realizing, ‘Oh, that’s what it is.’ ”

While it has not been the year anyone envisioned for Pimentel, the Sox are optimistic that he has positioned himself to return to Double-A in 2012, at an age (22) that would still be young for the Eastern League. Though he has endured a year in which his command and results have suffered hits, the Sox still maintain that his stuff still ranks among the best in the system.

Pimentel’s velocity has been as high as 97 mph. The club believes that some of the mechanical inconsistencies he suffered, especially in Portland, were a byproduct of the fact that he is still growing and filling out -- something reflected in his velocity bump.

Back in High-A, Pimentel -- who had struggled to command his fastball down in the strike zone in Portland -- showed an improved ability to leverage his heater and generate downward plane. As he did so, his changeup started to play up again.

And, whereas Pimentel has struggled to spin a curveball consistently in the past, the Sox had him work on a slider that has shown solid early returns.

“He throws it from a similar arm slot and hand position as his fastball, so it’s easier for him to repeat it,” said Hazen. “It still needs to develop, but we’ve seen flashes of a real power slider that comes right off his fastball.”

In short, Pimentel has used his time in Salem to accomplish exactly what the organization hoped he would. Because he went back a level, his year will inevitably be viewed as a disappointment.

But the Sox believe that his struggles represented a pothole rather than an abyss. As such, the organization believes that his year will ultimately be seen as little more than a momentary detour in his path towards the big leagues. 

“Now he’s back on track. Now we’re looking to get him back to Portland next year,” said Boles. “You just have to trust what you see -- the ability. Trust your eyes.

“Obviously, there’s going to be some struggles throughout guys’ careers. With Pimentel, it happened to be this year. Every player goes through that. With Pimentel, it just happened to be this year. But he’ll be fine. We’ve still got a major league pitcher on our hands.”

That perspective underscores the fact that any single moment in time, any individual season, offers an incomplete portrayal of the player development process.

Middlebrooks, Pimentel and Tejeda entered the year in a cluster, with Middlebrooks regarded as being behind the other two. This season, that hierarchy has been turned upside-down. In another year, it may flip again.

On Monday, in their respective season finales, Pimentel dominated for Salem, allowing two runs on two hits in seven innings. Tejeda went 2-for-4 with a homer for Portland. Middlebrooks went 1-for-3 with a homer for Pawtucket.

It was a promising end for all three. Whether the day will serve as a building block towards major league futures for all, or any, of the trio remains to be seen.

“You can’t be a frontrunner in player development. Players aren’t going to hit .300 every year. They’re not going to go 10-0 on the mound every year, so you’ve got to stay the course,” said Boles. “These guys have a lot of upside. These are guys that have a chance to be impact players in the big leagues.

“You have to be patient. You have to keep in mind that it’s not going to be a fast track with every player. There’s going to be some bumps in the road. As long as you understand that, keep patience, show them trust, they’ll get through,” he continued. “You have to look at the tools. You have to look at the body. You have to look at what the future performance can be in the major leagues. We like all three.”