Does Notion Thief replace card draw from replacement effects like Sylvan Library?

Notion Thief has a replacement effect (using 'instead') that applies whenever an opponent would draw a card — except during their first draw in their draw step. Sylvan Library's ability is also a replacement effect that modifies how many cards are drawn during the draw step. These two replacement effects can interact, and the order matters.

Sylvan Library replaces the single draw-step draw with 'draw three cards, then put two back.' Each of those three individual card draws is still a card draw event. If an opponent controls Notion Thief, each of those three draws happens while Notion Thief is watching — but Notion Thief only skips the very first draw during the opponent's draw step for the 'instead skip' clause. The second and third draws are additional draws, so Notion Thief fully replaces those: the opponent skips them and you draw instead.

The affected player chooses the order in which replacement effects apply (CR 616.1). If the Notion Thief player applies Notion Thief's effect to the additional draws generated by Sylvan Library, those draws are redirected. However, the Sylvan Library controller still controls which cards to put back, since the 'put cards back' part of Sylvan Library's effect is not itself a draw.

Concrete example: It's your opponent's turn. They control Sylvan Library. You control Notion Thief. Sylvan Library fires, replacing their draw-step draw with 'draw three.' The first draw is their normal draw-step draw — Notion Thief doesn't intercept that one. Draws two and three are additional draws; Notion Thief replaces each so your opponent skips them and you draw a card for each instead. Your opponent still resolves Sylvan Library's 'put two cards back or pay life' clause.

Key rules: CR 616.1 covers applying multiple replacement effects; CR 614.1 defines replacement effects; Notion Thief's exemption for 'the first draw during their draw step' is printed on the card itself and confirmed by Oracle text.