The Championship outfit, who have won two league matches this season, travel to Old Trafford for the third round clash on Wednesday night.
Asked if his side will take the game seriously, Mourinho said: "Oh yes, serious because it's football you know, it's football, it's competition, we take it seriously.
"Of course I'm not playing De Gea or Bailly but if we go to the next round probably these guys play and I give rest to others.
"Our squad is good but we don't have 30 players. We have what we have and a couple of injuries so we need some of the players that played [against Everton] to play and we need some to be on the bench because we need a strong team.
"I know the quality of the Championship and I know if we don't take the game seriously we are going to lose."