An enormous number of players have undergone Tommy John surgery -- over 80 percent of MLB games now feature at least one -- so we've been able to learn a lot about what to expect from a pitcher his first year back.
The pattern is clear -- a little wildness, often an uptick in velocity, but overall a slight regression from years past. Which is why Zack Wheeler's return has been so notable -- he's almost exactly the player he was prior to his surgery, despite (or perhaps because of) his exceptionally long layoff.
Thus far, Wheeler has been effective on the mound, with a good-though-not-great ERA of 3.