Alnwick, on loan from Tottenham, sustained a facial injury in the latter stages of the 4-1 Carling Cup defeat by Sunderland at Carrow Road on Monday night.
Manager Paul Lambert has already added cover by signing stopper Fraser Forster on a one-month loan from Newcastle, although teenager Declan Rudd could be in line to make his first-team debut.
The Canaries will definitely be without defender Michael Spillane after he was sent off for a professional foul against the Black Cats.
Jens Berthel Askou is expected to return to the centre of defence after being rested for the cup tie with Gary Doherty and former Pools captain Michael Nelson battling to partner him.
Midfielders Matt Gill (knee) and Owain Tudur Jones (calf) are sidelined while left-back Adam Drury should return after sustaining a knock in the 5-2 home win over Wycombe.