Louis van Gaal admits that Ashley Young's injury could force Manchester United to look for a new full-back during the final week of the transfer window.
The United manager confirmed on Friday that Young has suffered a "severe" groin problem which will require an operation, meaning the 30-year-old is set for a lengthy spell on the sidelines.
With Luke Shaw, Marcos Rojo and Antonio Valencia all recovering from long-term injuries, Van Gaal's options at left-back are now limited to Daley Blind, Matteo Darmian or young Cameron Borthwick-Jackson.
And the Dutchman admits he is now seeking reinforcements before the transfer window closes on February 1.
"I think when you see how unlucky we have been in the full-back position," Van Gaal said on Friday. "I have started with Valencia, Shaw, Rojo, Darmian and Young, sometimes, and they are all injured - only Darmian is now back.
"So, I need full-backs and you have seen how I have used youngsters like Borthwick-Jackson and [Guillermo] Varela who are doing great but the level of Manchester United is high.
"To give every week a high performance, normally it is very difficult to deliver that for youngsters. That is why we have to look to replacements. The difficulty is the clubs don't want to sell in this period. We are looking but it is not so easy."
He added of Borthwick-Jackson: "He's more or less a very good player for his age. He has shown already that he can cope with the pressure but, with youth players, the question is always: are they consistent? One match or a substitution is always good, but consistency is always the question mark."
Van Gaal's side have enjoyed an upturn in form in recent weeks, winning three of their last four games, including last Sunday's clash with Liverpool.
Wayne Rooney has scored five goals in his last four games and Van Gaal believes his captain's recent displays have been key.
He continued: "It is a fact that since Rooney is scoring we have won our games, or at least we draw against Newcastle United away. There is a parallel and I have said that always that he is not only important as a player but as a captain, so I am very happy with him."
Van Gaal also praised his players for the heated dressing-room exchange at half-time of last week's win at Anfield, which was confirmed by Morgan Schneiderlin earlier this week.
"I think not only Wayne Rooney is doing that, there are several players doing that and that is what I want," he added. "It is not only the manager, that's also why I have made this approach to the players."
