The former Manchester United striker returned in February following a nine-month injury lay-off, and Wenger believes he quickly became fatigued:
"After they come in they are super sharp and then after, against Crystal Palace, he had a very average game and looked a bit for one or two games like he had paid for the physical energy he had given."
Arsenal have struggled in recent months, and the performances of Olivier Giroud have typified their season. Wenger, however, has persisted in starting the Frenchman in recent weeks, but it was only when Welbeck was brought on against Norwich City that good things started to happen for the Gunners.
The Arsenal boss feels that Welbeck has now regained his full match sharpness, after easing him back over the past few weeks:
"I let him rest one or two games and you bring him back in against Norwich, when he came on, you could see he is a different animal again."
The North London outfit look likely to secure a top four finish again with disappointing results for both Manchester clubs yesterday improving their chances.
They head to the Etihad on Sunday to take on Manchester City, and will be looking for a victory to secure a third place finish, and avoid having to play a qualifying round to enter the Champions League.