Alnwick was on the bench for the game.
"I kind of knew straight away because Robbie (Elliot) got injured and he wasn't getting a scan until Monday morning," the youngster explained to nufc.co.uk. "They spoke to me straight away and Woody said 'make sure you're ready because you're going to be on the bench.'
"I'm not one to get really nervous or anything like that but a couple of thoughts are going through your head; if anything happens, you have to come on."
"It was funny because Harps actually said to me after the game, he thought I was on," 19-year-old Alnwick smiled.
"He clipped him, he said, and if he had gone down he was off. It was a bit heart in mouth but you've always got to be ready as a goalkeeper and I felt like I was ready.
"I'd had a good warm-up and I feel like I'm ready for anything that's thrown at me."