Fernandes was unable to lead United to Europa League success as they were undone in the final on penalties by Villarreal.
The match itself was a low quality affair with the 26-year-old unable to influence proceedings.
Wenger told Bein Sports: “I personally feel that he finished the season on his knees, and he missed the spark. And sometimes you can see as well, I found him much more nervous in the game.
"So that as well a sign of fatigue. Your top-level is calm, concentration, control. And in the last four or five games, I find him a bit weak on that front. So I hope that he can recover to be efficient for Portugal."
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