United were without injured duo Nemanja Matic and Ander Herrera for their Friday night opener but Pogba started his first match since the World Cup final and opened the scoring in a man of the match display.
"Was not hard," Mourinho said. "The decision belonged to him because Andreas Pereira comes from Valencia bench as a winger to play number six, Fred, 10 days training, Ukrainian league, comes direct to Old Trafford, Ander Herrera injured, I had only two options: Scott or Paul.
"But if I play Scott (McTominay) I play two kids and a new player, so I asked Paul and he put himself available for the team and he was very, very good for us.
"In my opinion it would be a good performance but with the circumstances I think very good because pre-season was very hard and we needed this fantastic spirit by the players to manage this performance and this result.
"Of course some fantastic performances, the boys with the proper pre-season were in a much better condition. Luke Shaw, Andreas Pereira, Eric Bailly, Matteo Darmian, these boys were comfortable to play 90 minutes but the others were fantastic.
"Marcus Rashford was in trouble but he played, Pogba was a monster, we thought maximum 60 minutes but he managed the 80 and then Lukaku and Fellaini available to help the team.
"Fantastic spirit and I think periods of very good football and in the periods where Leicester were stronger we played as a team and we coped well with the situation. I think good three points, good victory, tomorrow morning training and free Sunday deserved."