The 21-year-old goalkeeper came on at half-time in Newcastle's impressive 2-1 win over Premier League leaders Chelsea after Rob Elliot was taken off injured.
Elliot was a stand-in for first-choice Tim Krul - currently out with a twisted ankle - but Alnwick now stands as the only fit goalkeeper available to Alan Pardew for the weekend's trip to the Emirates Stadium.
With injuries mounting, Pardew also revealed captain Fabricio Coloccini and Moussa Sissoko are likely to be unavailable.
"Jak will be in goal against Arsenal," Pardew explained.
"We'll have to dig deep for the Arsenal game. Colo looks like he has an injury and Moussa won't play. There's no Robbie Elliot either.
"But everybody is producing. You can only do that if you have a good team spirit at the training ground and a love for each other and a togetherness.
"It's about putting cover on for each other and they have done that.".
The result at St James' Park, secured by substitute Papiss Cisse's double, ended Chelsea's unbeaten start to the campaign
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