Pogba played a key part in France's successful World Cup campaign and was therefore allowed to miss most of United's preseason, FourFourTwo magazine reported.
The midfielder returned on Monday and, although he has not featured in any preseason friendlies, he was thrust straight back into the action four days later as the Premier League got underway once again at Old Trafford, United winning 2-1 on Friday.
Wearing the captain's armband, Pogba got the scoring started with a third-minute penalty and generally impressed in the center of the pitch, but it was his ability to keep going for 80 minutes despite a lack of a preseason that Mourinho focused on in his post-match news conference.
"Of course the boys with the proper preseasons, they were in much better condition," Mourinho said.
"[Luke] Shaw, Andreas Pereira, [Eric] Bailly, [Matteo] Darmian, they were comfortable to play 90 mins. The others were fantastic.
"[Marcus] Rashford was in trouble, but he played. Pogba was a monster. We thought [he could play a] maximum 60 minutes, but he managed 80.
"Then Romelu Lukaku and Marouane Fellaini were available to try to help the team. It was fantastic spirit, and there were periods of very good football.
"In the periods Leicester were stronger we played as a team and coped well."
Although some were surprised to see Pogba in action again so soon after his post-World Cup break, Mourinho insisted it was not a difficult decision to play him because Pogba said he was ready.
"[It] was not hard," Mourinho added. "The decision belonged to him, because Andreas Pereira comes from Valencia bench as a winger to play number six; Fred had 10 days training, Ukrainian league, comes direct to Old Trafford; Ander Herrera injured.
"I had only two options: Scott [McTominay] or Paul, but if I play Scott 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."
Pogba said he will always give his best to the Manchester United fans and his teammates "no matter what's going on" in a curious Instagram post following Friday's victory.
Pogba uploaded a photo with the accompanying caption, "I'll always give my best to the fans and my teammates no matter what's going on #pogfeelings".
"I won't be full from a World Cup," Pogba told Sky Sports. "I know it's a great cup, it will stay in my mind, but I want to win all the trophies. I haven't won the Premier League, I haven't won the Champions League, I haven't won the Euros so there are more to win.
"That [the World Cup] is in the past, it is part of history. I am someone who likes challenges and wants to progress and you have to leave it behind now.
"You have to keep it in mind, but you have to keep working to get better and better because I am still quite young. I want to progress and get better again and I want to win more trophies.
"We should start the Premier League with a win and we did today. We knew it was going to be difficult because some of us, including myself, didn't have any pre-season games.
"I came back on Monday so it was very difficult for the legs and the stamina but we played very well. We just want to go up and that's it."