The Brazil international midfielder found himself at the centre of a transfer tussle before and during the summer window.
Pep Guardiola had identified him as a top target in January, as revealed by Goal, but no deal was done.
Instead, the 25-year-old ended up making a £52 million ($68m) move to Old Trafford, with Mourinho proving to be the decisive factor in negotiations.
“That's true, I received an offer from them [City] and even talked to my Brazilian team-mates who play for our neighbours,” Fred told FourFourTwo.
“They were trying to convince me to join them, but in the end it didn't happen.
“Then it was the summer and, as I said, there were other offers, including from United. I quickly decided that was the right move for me.
“Mourinho is a top, top professional and every player likes to work with such a winner. I'm learning things from him, that's for sure. He can be quite strict, but that's part of his job.
“He's also friendly and funny off the pitch, and jokes with us in the dressing room. Jose was crucial in my decision to join Manchester United and I'm grateful for his interest in my football.”
Fred has endured a baptism of firm in English football, with United’s early struggles in 2018-19 making life difficult for all concerned.
He has, however, taken in 10 appearances to date and opened his goal account for the club – with an effort registered against Wolves seeing him become the 500th player to find the target for the Red Devils.
The hope is that there will be many more to come, with United having rediscovered their spark of late to record three successive victories across all competitions.
He will be desperate to come back into Mourinho’s plans, with a derby date at the Etihad Stadium on Sunday set to present himself with a chance to show City what they missed out on.