Louis van Gaal has admitted to his concerns over the injuries sustained by Rafael and Luke Shaw in Manchester United's 2-0 FA Cup third-round win over Yeovil Town on Sunday.
Full-backs Rafael and Shaw were both replaced at half-time at Huish Park, as goals from Ander Herrera and Angel Di Maria sent United into the fourth round.
Rafael has a suspected fractured cheekbone while England international Shaw sustained an ankle injury, casting their availability for Sunday's meeting with Southampton in doubt.
"Rafa has a fracture, so it's not looking good, and Shaw has an ankle problem," manager Van Gaal told MUTV. "I had to change, and I don't have more defenders, so I have to change the shape also.
"Rafael had a fresh injury so it was fantastic that he had the guts to play through. He has a fantastic mentality, it's unbelievable. But I had to change him and Shaw because they aren't 100 percent fit.
"I put a midfielder extra in and, because of that, maybe we had more ball possession in the second half, and we had more patience.
"We have to change from five to four, we played four in midfield in the second half, then we played better and we more patient, we kept the ball better also and, because of that, we could score the winning goal."
Although defender Patrick McNair played on despite having his head stapled during the match, there was more positive injury news for Van Gaal.
"We can be very happy because the players are coming back now," the Dutchman added.
"Angel Di Maria is coming back, Daley Blind is coming back, Marcos Rojo is coming back -- so we have more left-footed players, and that we need also. I have to solve every week another problem, and now during a match."