Bevis Mugabi will be out for four to six weeks with a dislocated shoulder.
The defender, who has established himself as an important figure in the Motherwell backline, suffered the injury on his return to training earlier this week.
“It is a blow to lose Bevis,” manager Stephen Robinson said.
“He has played his way into the team and acquitted himself very well.
“He just seems to be unfortunate that when he gets a run of games, he suffers an injury.”