Banta-Cain had the problem toward the end of last season. He was told to monitor it during the off-season, given the Patriots medical and training staff couldn't treat him during the lockout. When he ramped up his activities, it got worse. So he underwent surgery Friday to fix what was described as a tear.
Banta-Cain had five sacks last year and 10 the year before, so any impact on his regular season will be a blow to the Patriots' pass rush.
This is especially poorly timed considering how little preparation teams have before the season anyway. Hopefully Banta-Cain will spend his rehab studying up and watching film, or else even if he comes back in Week 1, he'll be way behind.