No, there are not certainties in sport. There is no guarantee that he wins another slam even if healthy. But I agree that it's probably more likely than not.
I agree nothing is certain in sports, but I'd most certainly not bet against Del Potro win another Slam if healthy, he's too good not to. Even after enduring wrist surgery and picking up injuries every other week since his comeback, it takes the very best players in the world to kick him out of most big tournaments. If he can finally overcome his injury issues (I admit it's a big if considering his history) I'd definitely expect him to add to his Slam tally.
Even if he keeps being this injury prone, chances are he might win at least another one at some point with a draw opening up for him, because who has been stopping him? Only a handful of players really, two of them - Federer and Nadal - unlikely to be a factor at the very top for many more years (Federer because of age, Nadal because of his wear and tear will probably focus more on clay and not on the hardcourts).