Sanchez will face former club Arsenal on Sunday, having left the 'Gunners' for his current employers in January.
His early form in Manchester came under fire, but the Chilean's performances have picked up in recent weeks, with the 29-year-old's header helping United come frorm behind to beat Tottenham in the FA Cup semi-finals.
Speaking ahead of Sunday's game, Mourinho told the press that having watched Sanchez play for years, he always knew he would be a success at the club.
"He was playing in our competition for years," Mourinho explained. "We all knew how good he was. That's why he was chased by many teams, I believe, and why we fought to have him here.
"Since day one, I was sure he was the right player, not just by the typical football qualities but also by his mentality. We needed that; a little bit more experience, mental aggressiveness and I knew sooner or later were coming.
"I don't like the January market but it was now or never. Now if we can have good Alexis until the end of the season then better for us. We need seven points and we have a Cup Final to play, and can he help us, I hope so. Next season, after pre-season, after being comfortable and adapted, he's going to be great for us."