Vlaar told the club's official website: "We have to stay strong now and we have another game which we need to prepare for.
"Of course, it is going to be a difficult game but every game is difficult. We will go and try to win the game.
"We can play well as a team, we know this, but we didn't do it in the second half against Southampton and that's disappointing for everybody - the players, the manager, the coaching staff, and of course, our fans.
"Everybody wants to do better and I hope we can show this in the way we play against Manchester City."