You're probably right, he was bought as cover because of the injury to Welbeck as well as Goroud's lack of form.
But the problem is Perez is the latest in a long line of strikers Wenger has bought that are just cover.
Perez is a decent player, and hasn't looked out of his depth at this level, and considering our lack of striking quality, he still can't get a run in the first team.
When Wenger spends as much money as he did on Perez, he usually scouts those players for ages, so he must have wanted him for the first team or he wouldn't have bought him.
This ranks as one of the most bizarre transfers Wenger has ever made, considering what he cost.