Manchester United manager Jose Mourinho said players need to "go to the limits" for the club.
Mourinho revealed that defenders Luke Shaw and Chris Smalling did not feel fit enough to face Swansea City.
The withdrawals of left-back Shaw and centre-back Smalling compounded United's defensive problems, with right-back Antonio Valencia and centre-back Eric Bailly having already been ruled out.
That meant Mourinho had to field a makeshift defence including Ashley Young at right-back and Matteo Darmian at left-back.
Phil Jones started in central defence after 10 months out with injury, playing alongside Marcos Rojo.
United secured a 3-1 victory at the Liberty Stadium and Mourinho told Sky Sports: "Smalling doesn't feel that he can play 100 percent with his pain. Luke Shaw told me this morning that he was not in the condition to play, so we had to build a defensive line."
He later questioned the bravery of some players without naming them, saying: "There is a difference between the brave, who want to be there at any cost, and the ones for whom a little pain can make a difference.
"If I were to speak with the many great football people of this team, they will say many times they played without being 100 percent. For the team.
"For the team, you have to do anything. That is my way of seeing it. Of course, it is not just the players, it is the people that surround the players.
"We have players with problems -- but we have, on the pitch, players with problems. In every sport, when they play at the highest level, see how many times they play and are not 100 percent.
"I have a friend who is a big tennis player and he remembers more the times he played with pain than the times he played without any pain, and that is what I mean.
"To compete, you have to go to the limits. It is a cultural thing for some. That is not my culture."
Asked whether some players had to change their approach, he said: "Exactly. More than for me, more for Man United."
Paul Pogba limped off during the Europa League defeat at Fenerbahce on Thursday but started at Swansea and produced a fine strike to open the scoring.
He told MUTV: "When you go with your head, everything follows. I was fit enough to play. The gaffer put me in and I just play my best every time."
Zlatan Ibrahimovic scored the other two goals for United, ending his six-game goal drought in the Premier League, and Mourinho said: "I knew that Ibrahimovic would score. It was always a question of time."
Discussing the Swede's booking, which means he will be suspended against Arsenal after the international break, Mourinho said: "I did not see it. I don't comment on referees."
The Portuguese said he felt "the players believe in me and I believe in the players".