The Mexico international blossomed under Sir Alex Ferguson but neither David Moyes or Louis van Gaal rated him.
Van Gaal ultimately flogged him to the Bundesliga, choosing to spend £36million on Anthony Martial as his replacement.
Mourinho has previously admitted he would never have sold Hernandez had he been in charge at the time.
And the former Chelsea boss believes the ex-Red Devil would have 20 goals for the season already under his guidance.
“We have some players that are not really players in love with the goal,” he said.
“They are good players, they are creative players, they can create but, naturally, they are not the kind of guy that is a killer.
“I give a simple example. In the way we play at Old Trafford, in the way we dominate opponents, in the way we play in the box, I think Chicharito [Hernandez] would have 20 goals easy.
GETTY
Javier Hernandez left Manchester United for Bayer Leverkusen in 2015
GETTY
Javier Hernandez won the Premier League with Manchester United in 2015
"Even coming from the bench for the last 10 or 20 minutes, he would have 20 goals.
"He is the guy that, naturally, the ball comes here, rebounds here, rebounds there - boom, goal."
Mourinho also praised Hernandez back in December.
"I wouldn’t mind to be at a club with great expectations but to have Ryan Giggs and Chicharito [Javier Hernandez], I think [Nemanja] Vidic still in the team. [Patrice] Evra. I wouldn’t mind," he said.
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