The 25-year-old Mitchell had his knee examined by doctors on July 23. The Boston Herald and Providence Journal reported at the time that the evaluation included a procedure to determine the extent and cause of discomfort in his knee.
Mitchell, who was a fourth-round selection of New England in the 2016 NFL Draft, had a decent rookie campaign with 32 receptions for 401 yards and four touchdowns. He had an expanded role in the second half of the season, reeling in all of his scoring passes over the final six games.
The 6-foot-1, 200-pound Mitchell also came up big in the dramatic, come-from-behind win over the Atlanta Falcons in Super Bowl LI. He had six receptions for 70 yards as the Patriots rallied from a 25-point second-half deficit to post a 34-28 overtime victory over the Falcons.
New England's wide receiving group has been in disarray this offseason, as the team traded Brandin Cooks to the Los Angeles Rams and saw fellow wideout Julian Edelman handed a four-game suspension for violating the league's substance abuse policy.
The Patriots have holdovers Chris Hogan, Phillip Dorsett and Kenny Britt to go along with offseason acquisition Cordarrelle Patterson, as well as Braxton Berrios, who was selected in the sixth round of the 2018 NFL Draft.