Rob Gronkowski underwent another procedure on his left forearm on Monday afternoon, and according to multiple reports, the surgery was a success.

The Patriots added a pair of players to their roster on Monday, signing wide receiver Mark Harrison and kicker David Ruffer, according to the league transcrion wire.

Harrison is a 6-foot-3, 231-pounder from Rutgers who had an up-and-down career as a collegian — in 2010, Harrison had 44 catches for 829 yards and nine touchdowns, which included a game against Cincinnati where he finished with 10 catches for 240 yards and four touchdowns.

Tom Brady

The Patriots opened organized team activities Monday — NFL-speak for full-squad, low-intensity get-togethers that will give us an opportunity to see the rookies and (many of the) veterans on the field together for the first time. (The media will have access to Tuesday’s workout.)

With the understanding that it will be impossible to make any wide, sweeping judgments of substance on any player because of the slow-speed nature of things (and with the knowledge that, at least according to reports, linebacker Brandon Spikes isn’t there), here are eight things we’re going to be keeping an eye on when we hit Foxboro Tuesday morning.

Tom Brady: The whole thing begins and ends with the quarterback, and while we don’t expect to necessarily be surprised by anything he might be doing, it’s always interesting to see where he is, both physically and mentally. One thing to watch will be how he does when it comes to working with the new faces, particularly at wide receiver. Another will be to keep an eye on his mechanics and any sport of tinkering he’s done with his delivery, something he discussed at great length with Peter King on Monday.

The rookies, specifically, Jamie Collins and Josh Boyce: We want to get a look at both of these guys because they’re both such athletic freaks, but Collins intrigues because he may end up playing more of a role in coverage, at least right out of the gate. As for Boyce, he missed rookie minicamp because of a foot issue, and as a result, this should mark his first time on the field with the rest of his teammates in an organized setting.

The tight ends, specifically, Jake Ballard: With Rob Gronkowski expected to be on the shelf at least through the spring, Ballard should certainly get plenty of reps at Gronkowski’s spot in the next month as he works his way back from spending the 2012 season on the sidelines because of a knee issue. It’s important to have a set of realistic expectations for Ballard — he not only spent the entire year on the shelf because of a knee injury he sustained in Super Bowl XLVI, he’s also joining a new system. Regardless, he’ll be interesting to watch. (In that same vein, we’ll also be watching linebacker Dane Fletcher and cornerback Ras-I Dowling, two other players who ended their season on injured reserve.)

New defensive linemen Armond Armstead and Tommy Kelly: The subsequent release of young defensive linemen Kyle Love and Brandon Deaderick mean that the two newcomers should see the bulk of the action when paired up in a four-man front next to Vince Wilfork. Armstead is a CFL export who brings some pass rushing skills to the table (he had six sacks last season with Toronto) while Kelly is a veteran defensive lineman who has also shown an ability to get after the passer. (He had a combined 14.5 sacks in 2011 and 2010.)

LeGarrette Blount: Not quite sure where Blount is at this stage of his career: Is he a 1,000-yard back who simply soured on playing for Greg Schiano in Tampa (thanks in large part to the emergence of Doug “Muscle Hamster” Martin?) and still has something left in the tank? Or is he a lost cause making a quick stopover in Foxboro on his way to somewhere else? (And what does all of this mean for Brandon Bolden?) We’ll get our first chance to make that determination Tuesday.

Undrafted/rookie free agents: The Patriots beat out several suitors for slot receiver T.J. Moe, while fellow WR Kenbrell Thompkins is a former JUCO star, and tight end Zach Sudfeld is a 6-foot-7, 255-pound behemoth who will be hard to miss on the practice field. They all face an uphill battle for playing time, but inevitably, at least one UDFA ends up making his mark on the roster as a rookie.

Special teams: It sounds like Julian Edelman will not be available, which means veteran return man Leon Washington will certainly get a chance to show what he can do with his new team as both the kick and punt returner. And while Zoltan Mesko remains one of the most unfailingly polite and genuinely good people in the game of football, he could be in for a positional battle this spring, as the Patriots signed Ryan Allen out of Louisiana Tech, who won the Ray Guy Award as the best punter in college football.

Brian Daboll: The former wide receivers coach — who spent the last few seasons working with the Chiefs, Dolphins, Browns and Jets — returns to the Patriots in a yet-to-be-defined role. While Daboll has no official title at this point, it wouldn’t be a surprise to see him spending time with the receivers. He was the receivers coach in New England from 2002 through 2006, which was the last stretch where the Patriots were able to successful draft and develop wide receivers. It will be interesting to see if he has a hands-on role or will be more behind the scenes when the team hits the field this week.

Chad Johnson was arrested Monday for violating probation. (Broward County sheriff’s office)

Former Patriots wide receiver Chad Johnson turned himself in to police and was arrested Monday in Fort Lauderdale, Fla., on charges that he violated probation. His bail was set at $1,000.

An arrest warrant was issued earlier this month after Johnson failed to meet with his probation officer and did not show proof of enrollment in a domestic violence education class. He has another hearing set for June 3.

Johnson, 35, is serving a year of probation following an altercation with his now ex-wife, reality TV star Evelyn Lozada. Johnson, who was known as Chad Ochocinco during his brief and unsuccessful tenure in New England, allegedly head-butted Lozada during an argument last August, one month after they were married. Lozada filed for divorce shortly thereafter.

Johnson was released by the Dolphins after the incident and has not played since.