The versatile Brazilian had been hoping to join the exodus of top talent away from the Ligue 1 champions during the summer transfer window.
Fabinho, though, was prevented from departing Stade Louis II, with Goal having revealed in July that he was identified as a target by United and PSG prior to them exploring other options.
The 24-year-old told Nice Matin : “During the window it was difficult because I was not sure if I would be staying.
“I was closer to leaving than staying.
“Playing in that context was not good. My head was elsewhere.
“Unconsciously, I was not giving my maximum. I was not trying to give a bad image, I could never do that.
“I was frustrated at the end of the window but I have turned the page.”
Having considered Fabinho to be a useful addition, United eventually ended up spending £40 million on Nemanja Matic as an alternative to their holding midfield poser.
PSG, meanwhile, turned their attention to luring Mbappe away from Monaco, with an initial loan agreement set to become a €180m transfer next summer.
Fabinho was prepared to fight for a move of his own at one stage, but admits he eventually made peace with his situation and decided against forcing the issue.
He added: “I could not forget about what the club had promised me.
“I was sure I was right and I thought about fighting for it.
“But I discussed it with my friends, my family, my agent, it was not an easy decision.
“The image it would have given was not me. Even if it was the only solution, I did not want to clash with the club.”
Despite wanting to move on, Fabinho has been a regular again for Monaco this season.
He has made 14 appearances in all competitions and remains under contract until June 2019.