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Louis van Gaal believes a mounting defensive injury crisis could wreck Manchester United's slim hopes of forcing their way back into Premier League title contention.
Ashley Young has become the fourth United full-back to be ruled out due to injury and the Dutchman admits it will be "difficult" to plug the gap prior to the end of the transfer window.
Young requires surgery on a serious groin injury and could be ruled out for the rest of the season, joining Antonio Valencia, Luke Shaw and Marcos Rojo on the sidelines.
"Ashley Young has a severe groin injury, it shall be operated on, so he's [out] for a long time," Van Gaal said on the club's official website.
It means the likes of youngster Cameron Borthwick-Jackson will have a big role to play as United look to shrug off their problematic start to the season and close the gap on leaders Arsenal.
Speaking in his press conference, Van Gaal said: "I have used youngsters a lot of times and they are doing great but the level of Man United is high and to give a high performance every week is very difficult for the youngsters.
"That's why we have to look especially in the transfer period for replacements, but the difficulty is we ask for a level, so we are looking but it is not easy."
To make matters worse for Van Gaal, neither Bastian Schweinsteiger nor Phil Jones have completed their expected recoveries and are ruled out of Saturday's home clash with Southampton.
Van Gaal must hope his side's shortcomings can be made up for by the kind of spirit encapsulated in the recent re-emergence of Wayne Rooney.
Five goals in the last six games have turned around Rooney's problematic campaign and shown the leadership qualities his manager requires to keep the club in Champions League contention.
Van Gaal added: "Since Rooney has started scoring we have won our games and drawn at Newcastle away, so there is a parallel.
"I have said always that he is not only important as a player but as a captain so I am very happy with him.
"I think it is not only Wayne Rooney - there are several players doing that and that is what I want because the players themselves are responsible.
"Responsibility for the result and commitment to get that result is very important. It is important that your team is showing that and they are showing it now."