Don't think there is any available players to replace Tevez in the Summer, so assuming he stays another season we'll have to take action next Summer
I'm not sure a top striker in good form will be available then either, at least not for anything less than what would surely be all of our budget + a huge wage increase. So maybe we finally transition to a 4-2-3-1/4-3-3 or something? Because if we can't even maintain the level of our attack when Tevez goes, we'd surely be better off implementing a change that means we don't necessarily need 2 strikers.
Hard to say though, considering we're trying to predict the best course of action a Year from now.
As for Dybala being a direct Tevez replacement, I wouldn't be happy. I think he MIGHT have it in him to be a top class #10 one day but we're now back in the European spotlight and we're not a developmental club so unless he hits the ground running it's not going to work well for him as a man for man replacement.
It's worth noting that even if a top striker is available this/next Summer and we can afford him, that he has to be more of a #10 because a traditional #9 no matter how good won't replace Tevez, they would merely be an improvement on Morata.