The third-choice stopper has been deputising for injured duo Tim Krul (ankle) and Rob Elliot (thigh), but is now a doubt himself after picking up a problem during Wednesday's 4-0 League Cup quarter-final defeat at Tottenham.
Alan Pardew could now be forced to play 17-year-old Freddie Woodman when rivals Sunderland visit St James' Park.
However, the Newcastle manager is hopeful the Premier League will allow him to make an emergency loan signing outside of the transfer window.
"He [Alnwick] has an injury to his shoulder so we're going to have to see [if he can play]," Pardew said.
"It is something we might have to look at because there is no way that a 17-year-old that's our fourth-choice should be allowed to go in goal at this level.
"We will perhaps talk to the Premier League about Jak's situation."
Alnwick has played just 134 minutes of top-flight football, coming on as a substitute in Newcastle's impressive 2-1 win over Chelsea on December 6 before starting last Saturday's 4-1 loss at Arsenal.
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