The Ecuadorian has impressed this season for the Red Devils at right-back and Mourinho insists he always felt that would be his best position.
Valencia made his comeback from a broken arm against Arsenal in Saturday’s 1-1 draw at Old Trafford and Mourinho says he offers an extra attacking option.
"I tried to sign Antonio a number of years ago, when I was at Madrid," Mourinho told MUTV.
"Even though he was not playing right-back at the time, I thought he could be phenomenal in that position. As it was, United told me 'no chance!'"
"I am not surprised by his form this season. He has been clean of injuries, save for an operation on his arm which did not keep him out for long, and I have also been able to keep him fresh by resting him in certain Europa League games.
"I think he is the right-back to give us the maximum we can in terms of our attacking football."