United signed Sanchez from Arsenal in the January transfer window in a deal which saw Henrikh Mkhitaryan move in the opposite direction.
But the 29-year-old failed to find his best form after leaving Arsenal, scoring just once in his first 10 appearances for United.
Since the international break, Sanchez has improved with a goal and three assists in his last two games.
But Mourinho believes United fans will see Sanchezs best form next season.
Hopefully next season he will be better, which is what we think is going to happen, Mourinho told MUTV.
I am not a big fan of buying players in the winter market because of the period of adaptation especially with attackers.
But hes obviously improving and getting more comfortable with the team.
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