The striker limped off after he banged his knee against Burnley at the weekend and will have to undergo exploratory surgery to find out the extent of the problem.
It's another massive blow for Paul Struck who now is without a fit senior striker at the club, with Drew Talbot, Marcus Today, Deon Burton and Barry Corr joining the former Rangers hetman on the sidelines.
With the news that no loan strikers will coming in because of budget and availability problems, MacLean could be out for around three months if anything serious is found when he goes under the knife.
Wednesday's squad is also missing Steve Adams, Richard Wood, Yuan Folly and Peter Gilbert - as well as the suspended Lee Bullen.