Morrison, one of Royals' stand-out players this season, damaged his ankle in Saturday's 1-0 defeat at Southampton.
The burly centre-back left St Mary's on crutches and was in a race to be fit for the crucial, rearranged clash at Stadium of Light.
Morrison, 21, could now join a growing injury list at Reading with goalkeeper Alex McCarthy out for the season and Pavel Pogrebnyak, Jimmy Kebe, Chris Gunter and Jem Karacan all recovering from knocks.
Royals boss Brian McDermott, said: "We've been unfortunate with injuries, I'm not one to really moan about it but we've had long term ones with the likes of Alex McCarthy. But that's why you have a squad.
"I'm not sure Morro will be available, it's his ankle and he's on crutches as a precaution. Pavel is out, Jem, Jimmy and so on. But come Tuesday we'll be ready again."
If Morrison is unavailable his place is likely to be taken by last season's Player of the Year, Alex Pearce.
However, there is some good news for Royals with former Newcastle United midfielder Danny Guthrie now fit and available for tonight's game.
Meanwhile, Black Cats boss Martin O'Neill could be without striker Steven Fletcher who missed Saturday's 3-1 defeat by Chelsea with an ankle injury.
Sunderland skipper Lee Cattermole is also unavailable as he battles back from a knee ligament injury.
While McDermott is eager to play down the significance of tonight's match, most critics see it as a Premier League relegation six-pointer between two teams in the bottom three.
A win for Reading would leave them three points from safety while a defeat for McDermott's men would leave them six points adrift of Wigan and enable Sunderland to climb out of the drop zone.
McDermott added: "We've got the opportunity to put things right and that's really, really important to us.
"I understand that the supporters want to see their team win games. They stayed with us again on Saturday and I thought they were fantastic.
"They will have been as frustrated as we were, everyone was bitterly disappointed. But, I'm sure they'll travel up again on Tuesday, this is the time where everyone wants to be together and do what we have to do for our club.
"We were gutted with the result at Southampton but in football games come around very, very quickly. I'm resilient, the players are resilient - and you have to be in this game.
"We will get on with it, do our stuff and make sure that we're difficult to play against first and foremost. It's a home game for Sunderland, we've got to go there and play, do our stuff."