Lynn limped off the field after a pitch during the fourth inning.
The 34-year-old had issues with the same knee late last season and went on the injured list for 10 days. He also spent almost a month after the season resting and rehabbing the knee.
Lynn's injury comes hours after Chicago announced that left-handed reliever Garrett Crochet will need season-ending Tommy John surgery.
Lynn went 11-6 with a 2.69 ERA in 28 starts last season and finished third in AL Cy Young Award balloting behind winner Robbie Ray of Toronto and Yankees ace Gerrit Cole.