His last league goal was on March 31 and he has scored just three times since making the move.
On Saturday, Sanchez travelled with the United squad to West Ham but was surprisingly not among the 18 players selected to feature.
And Mourinho claimed the decision was based on “Options, just options.”
But according to the Independent, Sanchez’s time at United has turned sour.
Alexis Sanchez's form at Man Utd has been poor (Image: GETTY)
And the report says there are sources at Old Trafford who feel Sanchez may be “burned out” and that Arsenal pulled off a “business masterstroke” by offloading him.
Speaking earlier this month, Mourinho said he remained hopeful Sanchez would rediscover his form.
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Jose Mourinho has been unable to get the best from Alexis Sanchez (Image: GETTY)
"He had a good pre-season, he had a good start to the season.
"Then he had a small injury that kept him out and then he came just one day before Tottenham, and he comes into the game with the result 2-0. Let's see from now.
"He is committed, he is focused. He works hard. I believe he will improve."