Motherwell keeper Trevor Carson is out for an indefinite period after developing deep vein thrombosis while on international duty with Northern Ireland.
The 30-year-old won his fifth cap in last Sunday's Nations League game against Austria but returned with a blood clot in his shoulder and is under the care of University Hospital Wishaw, with no time frame on his return to action.
Manager Stephen Robinson said: "With my limited knowledge, he has had a blood clot and it is going to keep him out for a long time.
"He has been in and out of hospital since Wednesday. He felt a lot of pain while away with the international squad, came back and we thought it best that we take him into hospital
"The medical staff at Wishaw and our medical staff have been superb with him.
"There are still ongoing investigations so that we can get to the bottom of the problem and make sure he is on the right tablets and medication that gets him back as quick as we can, but he is out for the foreseeable future.
"First and foremost our concerns are with Trevor's health. I am sure he will be fine but it is now important that we get the right treatment.
"He was home last night and went in again this morning. We exchanged text messages and he seems OK.
"Obviously it is a fright. You don't expect things like that at all, it gave everybody a fright.
"He has a young family as well that has to be considered, so it is very important that we make sure he is right and not worry about the football side of things."
Robinson also revealed midfielder Chris Cadden will be out for two to three weeks, maybe longer, with a tendon problem.
The Northern Irishman is more than confident with deputy keeper Mark Gillespie, signed from Walsall in the summer, who will take over from Carson for the visit of Betfred Cup finalists Aberdeen on Saturday in the Ladbrokes Premiership.
He said: "I signed two number one keepers and I have a lot of confidence in Mark Gillespie."
Robinson is looking for a response from his players after the "humiliating" 7-1 defeat by Rangers at Ibrox before the international break
He said: "We worked very hard. I think it was important that we got that out of our system.
"The scoreline was humiliating. Probably my most humiliating day as a manager and the players felt that as well, and the fans felt that, I know that for certain. That was a very hard one to take.
"We have to go and put in a performance. Aberdeen will all be playing for cup final places. We have to be on top of our game to get a result."