Krul is back on Tyneside and will continue to be assessed by United medics today.
Harper last appeared in a Premier League game in April 2011 for the Magpies in a 1-0 loss against Aston Villa.
Steve Harper: “I feel the best I’ve felt for a long time. We all knew that this season there was a chance to get games under our belts.
“I had a chat with the manager at the start of pre-season and he said there would be an opportunity. We all want to remind him we’re ready if needed.
“Having the extra games gives you that level of competitiveness. It’s good for everybody.”