For the Yankees – or any other team investing big money in a free-agent pitcher – the fear is that they get what A.J. Burnett produced on Saturday.

That doesn’t mean that New York or any other club is quaking about the prospect of a $16.5 million-a-year pitcher giving away a six-run lead in every start. The Yankees have every reason to believe that Burnett’s almost unfathomable inability to hold onto a lead that grew to a half-dozen runs after three innings was a rare – and perhaps one-of-a-kind – occurrence.

“You can expect that he’s going to (hold a 6-0 lead) most of the time. You really can,” said Yankees manager Joe Girardi after watching his team endure a stunning 16-11 loss at Fenway. “It just didn’t happen today for whatever reason.”

The concern is not so much this specific outcome, but instead what it represented. Burnett started out brilliantly, looking like a completely dominant pitcher through the first three innings. But suddenly, Burnett’s outing took a disastrous turn at that point.

Broadly speaking, that is the very reasonable fear of teams that make huge investments in free-agent pitchers. There is an unknown quantity that looms over the life of the contract in question.

A pitcher might dominate in the early stages of the contract, but history suggests that he will turn into a pumpkin by its conclusion, and often much sooner.

In that respect, Burnett’s outing against the Sox on Saturday served as a metaphor. The pitcher, who signed a five-year, $82.5 million deal over the offseason (a price tag that prevented the Sox from having anything more than cursory interest in him), displayed the ridiculous arsenal that inspired the investment.

The mid- to high-90s fastball, the devastating curve and the nasty changeup were all on display. But while that was good enough to allow Burnett to cruise through the first three innings, his excellence turned to dust far earlier than the Yankees could tolerate.

Burnett – whose maturity and focus had both been questioned during his career with the Marlins and Blue Jays – seemed stunned by the development. When the Yankees’ clubhouse opened to reporters, the pitcher was still dressed in uniform and staring into his locker, hours after his removal from the game.

He admitted that this defeat would be tough to digest, acknowledging that he was in for a sleepless night.

The pitch that Burnett lamented, over and over, was the first-pitch fastball to Jason Varitek with the bases loaded and two outs in the bottom of the fourth. Though the pitch was a 96 mph fastball, it sat squarely in the middle of the strike zone.

Varitek jumped on the elevated pitch, and slammed it into the visitor’s bullpen in right field for a grand slam that reduced the Yankees’ 6-1 lead to a 6-5 affair, immediately putting the Sox back in the game. In retrospect, Burnett said, he should have thrown a curve, rather than putting a fastball over the middle of the plate in a fastball count.

“I’ve just got to be smarter than that. That situation, you groove the first pitch like that, I mean, come on. If I had it all over again, I probably wouldn’t have thrown a heater,” Burnett said. “I’ve got to be better than that. I’ve got to be smarter than that. That’s not me. We’ll find a way to put this one behind us and get to that next one.

“(It was) bad pitch selection, I think,” Burnett added. “Bases loaded like that, you know he’s going to jump ship, and he did. That’s not thinking on my part. That’s unacceptable. The stuff I had out there today, and the offense we had today, the bullpen never should have been in that game. Bottom line.”

Burnett’s sudden collapse was all the more improbable because of his prior success against the Red Sox, particularly at Fenway Park. He entered the game undefeated in his eight career starts against Boston, going 5-0 with a 2.56 ERA, and 3-0 with a 0.40 ERA at Fenway.

All of that said, it would be ridiculous to draw conclusions about Burnett’s ability to fulfill his contract based on one terrible start. For that matter, even the entire 2009 season may prove insufficient for the Yankees to gauge their return on their $82.5 million investment.

Yet Burnett’s meltdown was a reminder that free agents who are paid based upon their past performances do not necessarily live up to those standards in their new contracts. The performance of pitchers in general is typically shrouded in some mystery. That notion is amplified for free-agent pitchers whose big contracts necessarily make them riskier propositions.

In a best-case scenario for a club that invests tens or even hundreds of millions of dollars in a pitcher, the failure to live up to expectations takes place for a single start. But history suggests something more sinister, namely the possibility that there could be years – particularly towards the latter stages of a contract of four or more years – when a pitcher is no more than a poor imitation of his formerly dominant self.

That was the phenomenon at work during Saturday’s tale of two games, when Burnett stunned his new club by allowing a career-worst eight runs.

“He’s the type of guy who can shut them down,” said Yankees outfielder Johnny Damon. “It just didn’t happen that way.”

“He was rolling. It was amazing,” said Girardi. “For three innings, it was really fun. A.J. was tremendous. He really was.”

The Yankees are optimistic that Burnett will recover quickly to emerge this season as  the top-of-the-rotation pitcher whom they signed during the winter. Nonetheless, a head-scratching outing like Saturday’s offers a reminder of the risks and questions associated with a big investment in a pitcher through free agency.

For how much of his five-year deal can the right-hander remain at that perch? Can Burnett become the freak – a pitcher like Mike Mussina, who signs and then lives up to a big-money contract? Or will he follow the trail that has been traveled by so many other talented free-agent pitchers, from Mike Hampton to Darren Dreifort to Chan Ho Park to Barry Zito, who ended up being a waste of tens of millions of dollars?

Only time will tell. Nonetheless, in the wake of Saturday’s defeat, the Yankees are no doubt might have to confront some doubts about one of their prize offseason acquisitions.