Mourinho admits he was willing to play Anthony Martial in their opening game against Leicester City but shunned against the move.
The French attacker was heavily touted with a move to join London-side Tottenham Hotspur this summer.
Anthony Martial endured an uninspiring campaign under the Portuguese coach last season, despite his blazing form at the start of last season.
Spurs were willing to make a swap deal with United. Toby Alderweireld would join the Mancunians, while Martial would head the opposite way.
But the Red Devils refused to budge and strongly insisted the Frenchman was not for sale.
United managed a narrow 2-1 against Claude Puel’s side in their opening game, with an early spot-kick and a late goal from Luke Shaw, before a consolation from substitute Jamie Vardy for the visitors.
Martial looked like he would come in for Paul Pogba, but Mourinho opted to slot in Marouane Fellaini instead.
He said in the post-match conference: “I wanted to play Martial and in a certain minute, a certain moment of the game, he was perfect to stretch the game get them in the counter and I was afraid to play him because I knew that Paul couldn’t resist for 90 and Fred would be in trouble.”
The Red Devils have won on the opening day in each of their past four seasons.