John O'Shea is expected to be out for five weeks and Manchester United team-mate Rafael is unlikely to return until after the international break.
The defensive duo suffered hamstring injuries during Tuesday's UEFA Champions League win over Marseille, with O'Shea forced off in the first half and his replacement Rafael hurt after half-time.
Both players will miss Saturday's Premier League game against Bolton Wanderers, with O'Shea's injury the more serious of the two.
The Republic of Ireland is out for the next month, while Brazilian right-back Rafael will be sidelined for up to three weeks.
The injuries add to United's defensive problems ahead of the Bolton game, with Rio Ferdinand potentially out for the season and Nemanja Vidic out due to a calf problem.
Jonny Evans could return after several weeks out through injury, but it is likely that Wes Brown will again partner Chris Smalling, with Fabio coming in at right-back.
"We're having a bad spell," said United manager Sir Alex Ferguson. "Only a few weeks ago we had every defender fit.
"We've got the bonus of Wes Brown being able to play centre-back and right-back, and Chris Smalling has been brilliant.
"Vidic won't make it. He is showing good recovery for this week and should be OK after the international break but not enough for Saturday.
"Hopefully Jonny Evans may be fit for Saturday, even if it is a bit of a risk. He has trained all week but he has been out for such a long time."
Ferguson could be boosted by the return of Park Ji-sung and Michael Owen against Bolton, while he offered updates on the fitness of three midfielders.
He added: "Fletcher is still suffering from a virus. Anderson is doing well running wise and getting close to a return. Owen Hargreaves trained all week which is good news.
"Park has been training and may be on the bench on Saturday. Michael Owen is available."