But the England striker proved his fitness by joining in with his Manchester United team-mates in a training session at the club's Carrington training ground today.
Rooney, United's 28-goal leading scorer this season, sat out Saturday's 1-0 victory against Wolves at Molineux after being diagnosed with inflamed kneecap tendons.
The problem, exacerbated by Rooney's 86-minute performance for England against Egypt at Wembley last Wednesday, is not understood to be a serious complaint.
Despite the low-key nature of the injury, though, United manager Sir Alex Ferguson criticised the 24-year-old for ignoring the injury and performing for England against the African champions.
Should Rooney fail to make the starting line-up against Milan tomorrow night, United, who go into the game with a 3-2 aggregate lead, would be left with limited attacking options.
Ferguson is already without Michael Owen, who is underwent hamstring surgery on Monday that has ended the former England forward's season.
And Bulgarian forward Dimitar Berbatov is continuing to play, despite the discomfort of an on-going knee problem that could require surgery at the end of the season.
United are already without the suspended Michael Carrick for the clash with Milan following his red card in the San Siro last month. And England defender Wes Brown is also out due to a broken metatarsal injury sustained in the win at Molineux.