The north Londoners extended their lead over David Moyes’ side on Saturday after beating Norwich 4-1 while a late goal at Old Trafford saw United drop more points as they were held to a 1-1 draw with Southampton.
While Wenger’s side lead the Premier League table by two points from Liverpool, United sit in eighth place having lost three times already.
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"It's too early," he told the club’s official website. "Eight points starts to be a little gap but they [United] are not out of the title race. And we are only two points in front of all the others.
"If you have a choice nobody wants to be eight points behind. If you have the choice, you want to be in the race at the top.
“But you cannot rule Man United out today. They have big players, big experience and they are a big club. It’s three games basically [to make up the gap] - that is very quick in our league."
Record signing Mesut Ozil has been a key feature of Arsenal’s superb start to the season and Wenger did not rule out adding to his squad again in January.
“We have a squad that, number and quality-wise, is very, very good," he added. "We are not desperate now [to sign anyone].
"I think it’s important, when you know who is out injured now, that they will come back in November and the beginning of December.
“Then you have a really massive squad. [But] if there is a good opportunity, why not?"