Wilbert Montgomery, the Philadelphia Eagles' star running back, will miss three to six weeks of competition because of the sprained right knee he suffered in Saturday's 27-14 exhibition victory over the Green Bay Packers, Coach Marion Campbell said yesterday.

Last week, the Eagles lost three players (linebackers Frank LeMaster and Reggie Wilkes and offensive tackle Dean Miraldi) for six to eight weeks with various injuries.

The Eagles alleviated part of their linebacking problem yesterday by acquiring Joel Williams from the Atlanta Falcons for an undisclosed 1984 draft pick and reserve Bill Cowher from the Cleveland Browns for an undisclosed draft choice.

The 6-foot-1, 220-pound Williams, a five-year NFL veteran, was at the Eagles' training camp in West Chester for a physical, then signed his contract. The Eagles began negotiations with Williams' agent after LeMaster and Wilkes were injured.

Williams finished last season with 36 tackles, 30 of them unassisted, and three sacks with the Falcons. His best season was 1980, when he was credited with 15 sacks.

The trade for Cowher, 26, is contigent on his passing a physical today. Cowher, 6-3, 225 pounds, is a veteran of three NFL seasons He started his pro career with the Eagles as a free agent in 1979. He was the last linebacker the Eagles cut during the 1979 preseason.