Mourinho insists too much should not be made of the price tag.
"You make a relation between what you pay and the club revenue, and realise that other clubs paying 20, 30, 40 [million pounds] is a much bigger thing than what Man United did," the boss told reporters.
"I just want Paul to forget that and to play his football. [Having had] no pre-season, he had a very good impact in the first game and it's normal that, after that, he has a little decrease.
"But I'm full of trust in him because I know the player he is; I know he's a very good guy with a lot of ambition, so the form will come naturally. As the team improves, form improves."
Mourinho believes Pogba is capable of playing both "positional" and more free-ranging roles in midfield, depending on the opposition and the resources at the manager's disposal.
"It depends on the situation," he added. "In the Premier League, we are playing with Wayne Rooney as a no.10 and [against Feyenoord] we played [Ander] Herrera in front of the back four with [Morgan] Schneiderlin and gave Paul more freedom to occupy spaces. It's not a problem with his position – he feels comfortable."