The Chicago Bears will be without Jim McMahon for at least four weeks after the quarterback injured his right knee in a 30-7 loss to the New England Patriots.

Dr. Clarence Fossier, the team physician, said Sunday the oft-injured McMahon would miss more than a month if a torn ligament or cartilage damage is found.McMahon was injured about two minutes into the second quarter on a third-and-9 play at the Chicago 23 line with the Bears trailing 9-7. He was hit from behind as he threw a shot incomplete pass.

McMahon, who completed two of six passes for four yards, did not play the rest of the game. He was replaced by Mike Tomczak, who completed 5 of 13 passes for 70 yards. Jim Harbaugh later took over and did not have a completion in five attempts.

McMahon missed four games last season with shoulder and hamstring problems, 10 in 1986 with shoulder injuries, three in 1985 with shoulder and neck problems, and two in 1984 with back and kidney ailments. He hadn't missed any games this season.