The German head coach's high-intensity brand of football needs extreme fitness levels and that means a step up even on last season's performances.
Wagner said: "Everything is quicker and more intense but I never had any doubt about our fitness or quality.
"We were one of the fittest teams in the Championship and I have no doubt we will be one of the fittest in the Premier League.
"We play a high intensity style of football and we need rotation too. We will stick to what we have done before."
"He was one of our priorities after promotion," said Wagner. "We signed him very early and as we started planning in the play-offs he always gave us the impression he would be willing to sign. He likes the club and the dressing room."
Mooy is the reluctant hero and Wagner added: "Aaron doesn't say much or smile a lot but he is very happy and proud.
"He deserves all the credit for the goal but also for his performance. He has great technical ability and defensive qualities. He is very unusual and that's why I like this player."
Wagner said he was delighted with the "Premier League atmosphere" which greeted his players, even as they got off the coach before the game.
He said: "It was proper Premier League. I asked for something extraordinary and the supporters delivered that. I am very pleased the players were able to pay them back with the three points. This helps us for sure."
Wagner admitted he didn't expect to start the campaign with two wins and two clean sheets and said: "If anyone knows that everything is possible in football it is this club after last season.
"Of course I did not expect it. We only want to be brave in every single game and we have done it so far.
"We are very happy we have put points on the board and we are not lucky, we deserved the points.
"This was a very strong performance from my team, especially from the defence. I am delighted with the workload of the players. Every player was responsible for defensive work."