Ritchie has been ruled out for at least two months with ankle ligament damage after being forced off in Wednesday's Carabao Cup defeat to Leicester.
Ritchie sustained the injury following a tackle from Leicester midfielder Hamza Choudhury, that Newcastle boss Steve Bruce described as a "horror challenge".
Asked about Ritchie's condition, Bruce said on Friday: "It's not great news. We think he's going to be maybe two months, eight weeks.
"There is ankle ligament damage too, bone bruising, cuts. The kid, as I said to you the other night, was a mess.
"Thankfully it's not six months, but it's bad enough when it's the best part of eight weeks."
"These tackles are the ones that damage you," he said. "Let's hope the young lad learns from it because he's a good player, the kid.
"But obviously, it doesn't help us and Matt in particular, who's going to miss at least a couple of months."