Mourinho is under fire after United suffered a second loss in three league games with their 3-0 defeat to Tottenham at Old Trafford on Monday.
Lucas Moura struck twice for the visitors in the second half, with Harry Kane's header opening the scoring after what had been an even first 45 minutes.
The result has seen Mourinho's future come into question, with the Portuguese demanding more respect as he stormed out of his post-match press conference.
Mourinho decided to use midfielder Ander Herrera in a defensive role in the defeat to Spurs, while Victor Lindelof came off the bench to replace Phil Jones and almost gifted Tottenham a goal with a short backpass.
Asked after the game if he knew what his best defence was, Mourinho accepted he was unsure.
"Why not? Because in the first game [it was Victor] Lindelof and [Eric] Bailly," he said.
"Today played [Phil] Jones and [Chris] Smalling, but now Jones is injured, and next match it will be Smalling, with another one.
"When Marcos Rojo comes he will be an option, and no, I don't know my best back four."
United will look to bounce back from two consecutive defeats when they take on Burnley on Sunday.