The 18-year-old was not named on the Man United teamsheet for any of their fixtures in August, even though he was included in Mourinho’s 26-man squad.
He is expected to play a big role for the Portuguese boss in September, where United will play six matches in 19 days.
Speaking about Mourinho, Fosu-Mensah claims there isn’t much for him to complain about under the Portuguese boss.
“It’s going well,” he said.
“I am fit and do not have much to complain about. I have not played much, but train with the first-team and do everything with the first-team. I want to fight myself into the base.
“I talk to Jose Mourinho about my development, which is going well. Mourinho has faith in me, so he keeps me in the selection.
Fosu-Mensah enjoyed an impressive breakthrough season under Louis van Gaal last campaign after showcasing his spectacular ability in the majority of his 10 appearances.
The Premier League season has just started, but we should see him in action soon as Mourinho has promised all his players game-time in September.
He has a bright future ahead of him and he will be relishing the opportunity to finally prove himself to Mourinho.