Butler had won back his place in the first team at Bristol City last Saturday but the 22-year-old came off injured early in the second half following a collision with an advertising board.
It was a mystery to boss Brian Laws what the injury was after the game on Saturday but it has since been established that Butler has injured his knee.
"Andy has had his scan and we are now waiting for the result. Until then there is no point in guessing, but we have to be optimistic," Laws told the Scunthorpe Telegraph.
Richard Hinds is likely to move back into central defence for the visit of Doncaster with Cliff Byrne returning at right-back.
Butler missed a month at the beginning of the season after picking up an ankle injury, and has struggled to regain the consistency that made him stand out as one of United's star performers last season.