Plymouth left-back Saxon Earley is set for three months on the sidelines as he requires ankle surgery.
Argyle boss Steven Schumacher confirmed the 20-year-old defender sustained damaged ankle ligaments in training.
"It is a big blow for him personally and a blow for us," Schumacher told BBC Radio Devon.
Schumacher also confirmed the Pilgrims have a deal in place to loan a striker, though it's unlikely to be completed before Saturday's trip to Birmingham.
Earley impressed on his Championship debut in the goalless draw at Watford before being ruled out of last Saturday's televised clash with Southampton at Home Park due to the injury.
Schumacher said: "It's not good news, unfortunately. He's torn ligaments in his ankle; it's going to require surgery, which is a blow for a young player who has just got in the team and got man of the match at Watford.
"Him being available in a televised game on Saturday was an opportunity to showcase himself and show everyone how good he can be, so it's unfortunate. He was growing in confidence and playing well.
"The bonus and positive is he's only young and the season is long - he'll be back. His op is booked in early next week, and as soon as that is done, he can get on the road to recovery."
Schumacher says he has strength in depth to deal with Earley's absence but is keen to complete one signing - bringing in a forward on loan.
He said: "Disappointingly, it doesn't look like he's going to be here at the weekend, but we have a deal in place. We are just waiting for the pieces to fall into place, and then he will be here.
"Until then, we have to be patient. If there's still no movement after Saturday's game, then we may be have to go to Plan B, but right now, I'm really positive we're going to get the player we want.
"I'm sure there'll be last-minute carnage [before the transfer window closes] next week, but hopefully we're not involved in it."