Yes, he's been a dud, but after only 21 at-bats, so was Carlos Gomez. It doesn't mean Yasmani Grandal needed PEDs to perform like he did as a rookie and wouldn't have been able to lift the bat from his shoulder otherwise. It just means his season has just started.
Granted, we'd have an easier time assessing him if he homered in his first game and doubled twice in his second, but rarely is baseball so convenient. Knowing his upside -- and yes, he was considered one of the best hitting prospects at the catcher position well before he ever failed a drug test -- he has the potential to be a game-changer at a relatively weak position. If Chris Iannetta or Erik Kratz is the best you can do off the waiver wire, then you should be willing to suffer through a couple down weeks for the sake of Grandal's potential.
What's a resonable expectation for him the rest of the way? Well, expecting some regression from last year but also accounting for the reduced size of the Padres' home park, I have him 10th among catchers in my rest-of-season rankings, ahead of notables Jonathan Lucroy, Jason Castro and, yes, Evan Gattis.