Celtic keeper Artur Boruc is facing knee surgery which could rule him out for at least three weeks.
However, an ongoing knee complaint requires an operation and he could go under the knife early next week which would mean him missing the Europa league trip to Hamburg.
But he will have to delay the surgery if Celtic cannot register back-up keeper Dom Cervi in time.
The American, who will be on the bench today, was not included in the Hoops' original 25-man squad submitted to UEFA, but the Parkhead club have asked for special permission to have Cervi involved in Germany on Thursday night.
Boss Tony Mowbray said: "Artur has a thigh injury which will take a couple of weeks to settle down. "He also has a slight problem with his knee and he'll need something done either immediately or in a few days.
"The over-riding issue is whether we can have another keeper in the squad for Hamburg. We are looking at the rules and we believe we can have a replacement as long as we can prove a keeper is injured. If we can get Artur's knee sorted now, he could hopefully be ready for the Dundee United game on November 22."