The Belgian striker had been out since September with a hip injury but came on in the second half at Villa Park and nearly found the net with a header.
"Christian has only trained four days in a month," PA quotes the Scottish coach as saying. "It is unfair on him to ask him to go straight in.
"He has trained with us a couple of days and a couple with Belgium. That wasn't enough to throw him into a game of that magnitude. It was never an option."