Consecutive 2-1 defeats have left their Champions League hopes and a fourth consecutive Premier League title in the balance and order must be restored quickly at home to Bayern on Wednesday if United's season is not to suffer a terminal crash.
Everything was running smoothly for United when Rooney put them ahead in the first leg of their Champions League quarter-final but the England striker deflected a 77th minute Franck Ribery shot past his goalkeeper Edwin van der Sar, twisted his ankle in stoppage time and was prone on the turf when Ivica Olic snatched a last-gasp winner for Bayern.
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Rooney missed Saturday's home league defeat by Chelsea which wrenched pole position from Alex Ferguson's men in the title race and while the red alarm lights are not flashing just yet, the fact Rooney may have been rushed back for the second leg against Bayern shows just how much United rely on their talisman.
United's late capitulation in Munich means they have to win, something they have managed just once in eight meetings with the Bavarian powerhouse - the epic 1999 Champions League final when they scored twice in stoppage time to claim a 2-1 victory.
"They're not the same team without Rooney," Chelsea boss Carlo Ancelotti said after his side coped comfortably with a toothless United side. "Rooney is a fantastic player who is difficult to replace with anyone else."
'GETTING OLDER'
Rooney's 34 goals this season in all competitions have disguised a shortage of firepower from other areas for the three-times European champions.
Their next highest scorer is Dimitar Berbatov with 12, followed by the injured Michael Owen who has just nine while the ageing Paul Scholes, such a reliable scorer of important goals down the years, has managed only six.
"United are easier to beat this season, a fact underlined by their seven Premier League defeats and that is down to a combination of three factors," Liverpool's former European Cup winner Alan Hansen said in Monday's Daily Telegraph newspaper.
"They are getting older, they are less resilient and quite simply, United are not as good as they were."
Rooney was initially expected to be out for three or four weeks but is undergoing intensive treatment and could play some part in Wednesday's return at Old Trafford.
While Ferguson is unlikely to start him, his mere presence on the bench would be hugely reassuring for United as they aim to overturn a deficit against a Bayern side full of confidence after taking top spot in the Bundesliga on Saturday.
Bayern, who had to battle hard to beat Schalke 04 at the weekend when they were reduced to 10 men for more than half the match, will arrive in confident mood and could also be boosted by the return of former Chelsea winger Arjen Robben who missed the first leg in Bavaria because of injury.
"I have hopes he'll be able to play, but we'll have to wait and see. His mus
