The pair were not present at Carrington for training with Ole Gunnar Solskjaer's first-team squad on Tuesday morning.
With both unavailable to face Barcelona, Fred and Scott McTominay are expected to join Paul Pogba in United's midfield.
Eric Bailly and Antonio Valencia were also absent at training, though Marcus Rashford returned after a minor ankle injury.
"We still hope maybe that Nemanja can be ready. Ander is probably out," Solskjaer revealed. "Eric and Antonio are definitely out.
Solskjaer is hopeful that Rashford will be ready to play at Old Trafford on Wednesday night, despite missing last week's 2-1 defeat at Wolves.
Rashford worked separately with a member of Solskjaer's coaching staff before joining the full group later in the session.
Alexis Sanchez and Matteo Darmian, both long-term absentees, were able to train on Tuesday but are unlikely to face Barcelona.
Provisional squad: De Gea, Romero, Grant, Dalot, Darmian, Jones, Lindelof, Smalling, Rojo, Shaw, Young, Garner, Fred, Gomes, Lingard, Mata, McTominay, Pereira, Pogba, Lukaku, Martial, Sanchez, Rashford.