Does Necropotence exile cards drawn from replacement effects like Sylvan Library?

Necropotence has a replacement effect (CR 614.1) that says "If you would draw a card, exile that card face down instead." This applies to any card draw, regardless of the source — including replacement draws generated by cards like Sylvan Library.

Sylvan Library creates its own replacement effect that modifies your draw step: instead of drawing one card, you draw three and then put two back (or pay life). Both Necropotence and Sylvan Library try to replace the same draw event. When two replacement effects apply to the same event, the affected player chooses the order in which to apply them (CR 616.1). However, no matter which order you choose, Necropotence's effect ultimately intercepts each individual card draw and exiles those cards face down instead of putting them in your hand.

Because the cards never reach your hand — they are exiled by Necropotence — you still drew three times in terms of rule tracking, but all three cards end up exiled. Sylvan Library's "put two back" instruction then fails to find cards in hand to return (or you can pay 4 life each to keep them, which is moot since they're already exiled). In practice, Necropotence plus Sylvan Library means you exile cards rather than drawing them into your hand, making Sylvan Library nearly useless in this combination.

Concrete example: You control both Necropotence and Sylvan Library. At the beginning of your draw step, Sylvan Library triggers and sets up a replacement to draw three cards. As each card would be drawn, Necropotence instead exiles it face down. All three cards are exiled; none enter your hand. You have no cards to put back, so Sylvan Library's "return two or pay life" clause applies to zero cards in hand drawn this way.

Relevant rules: CR 614.1 (replacement effects), CR 616.1 (interaction of replacement effects), and Necropotence's Oracle text.