The Hammers travel to Old Trafford on Wednesday still smarting from their 3-1 defeat at the hands of London rivals Tottenham at the weekend.
However, a solid start to their Premier League campaign has seen the East End outfit climb to an impressive eighth position under manager Sam Allardyce.
And the 23-year-old centre-half believes the club must put the loss against Spurs quickly behind them with games against United and then Chelsea and Liverpool at home to come.
"We didn't play to our best against Spurs, but we need to put it behind us quite quickly because we have another hard game against United and they come thick and fast and tougher as well," said Tomkins.
"But we have to go to Old Trafford full of confidence because we have got the points on the board and we can only get better after the Tottenham game.
"They have got a lot of pace with their wingers, but we can still go there with a lot of confidence.
"We will sit down with the gaffer and see what he says about how to deal with them.
"Against Manchester City at times, we were sitting back and letting them play in front of us and that day we were hard to break down. It might be a case of doing that again at times in the game, but we will just have to see."