Minnesota Vikings star Matt Kalil reportedly underwent arthroscopic surgery on both of his knees this off-season, as well as having injections for joint pain.
Kalil admitted in December that a knee procedure last summer had caused him to struggle during the season, and it could be a factor again ahead of the new campaign.
The 25-year-old continued to have treatment earlier this year, but the former first-round pick has failed to fully recover, according to USA Today.
Minnesota recently picked up the fifth-year option on his contract, but the organisation will remain concerned over his health.
The former Pro Bowl selection has never missed a start in three seasons, but his production has slipped in that time.
