Charlie Neibergall/Associated Press

Tom Brady and Rob Gronkowski aren't at the New England Patriots' OTAs. They are, however, reportedly together at Brady's TB12 clinic receiving treatment.

Karen Guregian of the Boston Herald reported the Patriots Pro Bowlers were at the clinic receiving treatment Monday and Tuesday.

The TB12 clinic has become a source of tension between Brady, Gronkowski and Bill Belichick, as the coach's two stars have increasingly distanced themselves from team trainers in favor of their own health regimen.

TB12 is run by Brady's trainer Alex Guerrero, who touts a number nonconventional training and diet methods. ESPN The Magazine's Seth Wickersham profiled the growing distrust between the three biggest faces in the Patriots locker room, with Guerrero's presence leading to tension between Belichick and Brady. That only continued to grow when Gronkowski began adopting some of the methods.

Guregian reported last month that Belichick "chastised" Gronkowski in front of teammates for being a TB12 client.

Brady and Gronkowski are also seeking new contracts from the Patriots, which has only served to exacerbate the already-festering tension.

The Patriots OTAs are not mandatory, but Brady has historically been in attendance. In 2013, Brady admitted that OTAs, while voluntary and non-contact, help carry over into the regular season.

"He (Bill Belichick) talks about, you think it's just an OTA in the spring time and it's not that important and all those things that probably could enter your mind," Brady told reporters at the time. "The truth is, this lays the foundation for the start of training camp and if you have a good training camp, it usually means a good start to the season. A good start to the season leads to good position entering the second half of the season. Everything ends up having some significance to it."