The answer is … no one.
No one can replace Malik Nabers within the Giants offense. No one can replace what Nabers provides to the passing game. No one commands the extra attention from the opposing defense that Nabers attracts on every snap. No one makes the Giants quarterback, whoever it is, feel emboldened to toss up a 50-50 ball and confident it will be a reception — perhaps a highlight-worthy one — the way Nabers does.
No one will force opposing defensive coordinators to spend hours in meeting rooms trying to figure out the best way to contain a special athlete the way Nabers made them put in the extra hours.
There is always a next-man-up mentality in the NFL, but with Nabers out for the rest of the season, there is no one remotely capable of being a Nabers facsimile in any way, shape or form.