The Gunners were beaten 3-1 by Jose Mourinho's side on December 2 and have gone on to draw their next two Premier League outings away at Southampton and West Ham.
Taking just two points out of a possible nine has left Arsenal down in seventh place heading into Saturday's visit of struggling Newcastle.
But Wenger feels there may still be a hangover from the way his side lost to United, who struck twice early on and had goalkeeper David De Gea to thank for making a number of quality saves.
"It still plays on their minds," he said when asked about the United defeat.
"You need to get that out of the system
When you have so many chances as we had and you come out with nothing, it's difficult to swallow and it takes a little while.
"I think the best way is to continue to put that behind us and focus on the quality of our game."
While De Gea's man of the match display helped keep Arsenal at bay for much of the contest, Wenger's forwards have been misfiring of late.
Arsenal have created more chances than any other team in the Premier League this season but have only managed to convert 13 per cent of them, while both United and leaders Manchester City boast a 23 per cent conversion rate.
Across the three games against United, Southampton and West Ham, Wenger saw his side have 66 shots on goal but only Alexandre Lacazette's consolation against the Red Devils and Olivier Giroud's late equaliser at Saints found the back of the net.
"At the moment there is a difference between points we get and the domination we have in the game," the Arsenal boss added.
"Of course it is down to the fact that we've played recently against two teams who defended extremely well at home
West Ham have shown that against City and against Chelsea.
"Maybe we have to speed up our passing and have more penetrating runs in the final third to create more chances
Maybe we need support for our central strikers in the box as well."
Wenger broke Arsenal's transfer record to bring in Lacazette in the summer, with the Frenchman having secured the best conversion rate across Europe's top-five leagues last season.
But he was left on the bench at the London Stadium, coming on for the final eight minutes of the 0-0 stalemate with West Ham.
Wenger is now likely to recall the 26-year-old against Newcastle after insisting it was injury which prevented him starting against the Hammers.
Asked if Lacazette was dropped, he replied: "No, (it was) because of rotation and he recently had a groin problem and I didn't want to push him too much.
"I don't know how you call that, when you rotate you rotate
I don't feel that (he was dropped), I just think he was rested."
Wenger also revealed he will be without midfielder Aaron Ramsey for up to three weeks due to a hamstring problem, while Theo Walcott is out with a groin injury.
Source : PA