Jose Mourinho did not know if Anthony Martial had a head injury when he left the player on the field during Manchester United's 3-1 defeat against Watford on Sunday.
Martial required medical treatment following a clash of heads with Watford's Daryl Janmaat after 26 minutes.
The France international was kept on the field then showed a lack of awareness as he was dispossessed and hurt by a challenge from Miguel Britos, leading to Watford's opening goal and prompting him to be substituted.
Speaking at his postmatch news conference, Mourinho said: "I had to be on the game, at that time, and not worrying with Anthony. I have not had time to see about the situation. It wasn't an action to go to hospital.
"I didn't know why he came off, whether he was injured in his knee, his ankle or his head. I just knew, in that moment, he was not in a condition to carry on. I just knew I had to change. I don't know yet what happened."
Mourinho blamed the defeat on "individual and collective mistakes" and was critical of referee Michael Oliver, while United legend Paul Scholes spoke out about the "sloppy" nature of the performance.