That's such flawed criteria.
Obviously, a player of Leonard's magnitude would be highly important to any team, but part of the reason he's so important to this one, is the lack of wing depth.
Even if Ginobili were healthy, they need him with the bench and none of the other options is a fit in the starting lineup nor all that appealing to begin with, so of course that exacerbates the loss.
The reality is, there is no race for the MVP. Curry is having a historically great season (impacting the game as much as possible for someone of his stature) and barring a significant injury, had it wrapped up 20 something games in.