Does a replacement effect that changes a draw to exile apply before a 'whenever you draw' trigger sees the draw?

Replacement effects (CR 614.1) modify or replace events before they happen. When a replacement effect changes a draw into exiling a card (such as the effect of Narset, Parter of Veils or a similar effect), the draw event never actually occurs — it is replaced entirely by a different event (exiling a card). There is no 'draw' for any triggered ability to detect.

Triggered abilities that use the phrase 'whenever you draw a card' (CR 603.1) watch for a specific event — a card being drawn. Because the replacement effect intercepts and replaces the draw before it resolves (CR 614.5), the triggering event (the draw) never takes place, and therefore the trigger condition is never met.

This contrasts with effects that trigger off the replacement effect itself or that watch for cards leaving the library by other means. Only actual draw events satisfy 'whenever you draw a card' triggers.

Concrete example: You control Erayo, Soratami Ascendant (which triggers when the fourth spell is cast this turn) — but more relevantly, suppose you control a 'whenever you draw a card, draw an additional card' effect, and your opponent has Narset, Parter of Veils in play. When you would draw a card, Narset's static replacement effect applies first (CR 614.5), replacing the draw with exiling the top card. The draw never happens, so your 'whenever you draw' trigger never fires.

Key rules: CR 614.1 (replacement effects modify events), CR 614.5 (replacement effects apply before the event occurs), CR 603.1–603.2 (triggered abilities watch for events that actually occur).