Matteo Darmian and Chris Smalling are facing spells on the sidelines after going off injured in Manchester United's 3-2 defeat at Wolfsburg on Tuesday.
Louis van Gaal confirmed Darmian picked up a hamstring problem and said Smalling appeared to have hurt his groin in the loss, which saw United exit the Champions League.
The Dutchman said Smalling's injury, which came after United had used all three substitutes, had been a key factor in Naldo's winning goal for Wolfsburg, which came just two minutes after Josuha Guilavogui's own goal had made it 2-2.
"Matteo Darmian has a hamstring injury and Smalling probably a groin injury," Van Gaal said. "Smalling could not jump at the moment of the set plays and that goal [Wolfsburg's third] was also because of that.
"He had to give it [defensive responsibility] to another player and that's always a question of communication and quality. With Naldo [who also scored the opener], we knew in advance that he was a very important header for them."
Van Gaal said he did not expect Darmian and Smalling to be available for Saturday's Premier League game away to Bournemouth.
"I don't know, it's happened this evening, but normally they are out for Saturday," he said.
United began the game at the Volkswagen Arena with the inexperienced Guillermo Varela at right-back and had to introduce 19-year-old Cameron Borthwick-Jackson as a replacement for Darmian due to an injury crisis that has also seen Phil Jones, Marcos Rojo, Luke Shaw, Antonio Valencia, Ander Herrera, Morgan Schneiderlin and Wayne Rooney sidelined.