The 26-year-old has been Arsenal's standout performer this season and has scored 18 goals in 32 appearances since signing from Barcelona for 35 million pounds in July.
But the Chile international went off with a knee injury in last week's 2-1 win over Leicester City and Wenger conceded that Sanchez may be showing signs of fatigue.
"He had not the same intensity in his game as he had until now," said Wenger, whose side are fifth in the Premier League table with 45 points from 25 games.
"I'm not sure he was free in his head to sprint as he used to. I don't know if it was down to the kick he got on his knee, but he was not himself.
"He has a swollen knee and he cannot move well, but it's only a kick, nothing else."
Arsenal host Middlesbrough in the fifth round of the FA Cup on Sunday before travelling to Crystal Palace in the Premier League next weekend.
(Reporting By Michael Hann; editing by Martyn Herman)
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