Manchester United will assess Robin van Persie's ankle injury on Monday as manager Louis van Gaal struggled to justify Radamel Falcao's omission from the squad for the 1-0 defeat to Southampton.
Dusan Tadic scored the only goal for Ronald Koeman's team as United were beaten at home for the first time since Van Gaal's competitive debut, despite the 63-year-old's assertion that his side dominated.
Top scorer Van Persie, who had struck twice when United won 2-1 at St Mary's in December, limped off after an hour and the club are waiting to discover the extent of the damage. "It is an ankle injury," said Van Gaal in his news conference.
Van Persie was replaced by midfielder Ander Herrera as on-loan Falcao, who had started their five previous matches, was not even named among the substitutes, which included three central defenders.
Van Gaal confirmed the Colombian is not injured but said his plans were affected by the need to replace Angel Di Maria, Luke Shaw and Daley Blind, all of whom were making their first starts after being injured.
He added: "I have to compare players in my selection and that is what I am doing when we train and then I have to look through the composition of my selection and what I can do during the match and what players I have.
"I know in advance I have to change Shaw and Di Maria. Normally I would change Di Maria for [James] Wilson because I know I want speed in attack. I know in advance I have to change Daley Blind because he was out for six or seven weeks and I take a risk with him [by keeping him on] because Van Persie was injured."
Van Gaal said he wanted to use Marouane Fellaini as a direct option, rather than have Falcao among the replacements, and knew he had the option of pushing captain Wayne Rooney into the attack.
He explained: "When you see the composition of my bench and I want to change the playing style with Fellaini, I don't have any position for a second striker [on the bench]. Also I have Rooney."
Van Gaal said Southampton played to get a point and felt his side were unlucky.
"We were the dominating team without creating chances," said Van Gaal. "They came for a draw and they got away with a victory. That is a little bit of luck [for Southampton] and unluck [for United]."