The Chile international is seemingly set to feature against his former club Arsenal, having made the move earlier this year as a swap deal with Henrikh Mkhitaryan.
Sanchez had a tough ask joining the United squad mid-way through their season, and after a string of somewhat mediocre performances, the forward has returned to form against City and Tottenham.
Mourinho said: ''We fought to have him here and I was, since day one, sure that he was the right player. Not just by the typical football qualities, but also by his mentality, and we needed that.''
He added: ''I don't like the January market but it was now or never. We needed a little bit more experience, we needed a little bit more mental aggressiveness, and I knew that sooner or later, we're coming [to improvements].''
The Portuguese coach concluded: ''We need seven points and we have a cup final to play, can he help us, I hope so. If we can have good Alexis until the end of the season, then better for us.''
Meanwhile over in the Arsenal camp, Sunday's match will have plenty of emotional value as it marks Arsene Wenger's final shot at glory against United with the club.