Ritchie was injured in the second half of his side’s 1-1 draw at Tottenham in the Premier League on Sunday, sustaining similar damage to the dislocation suffered by team-mate Fabian Schar against the same opposition in July.
Defender Schar has been on the sidelines since.
“He will probably have an operation today – I know he’s seeing the surgeon today or certainly in the next 24, 48 hours," said head coach Steve Bruce.
“It’s a very similar injury to Fab Schar, disruption of the joint, so how unfortunate are we?
“To have one is bad enough, but to have two in a matter of weeks is incredible.
“It looks as if he’ll need an operation and it will be performed very shortly.”
Asked how long Scotland international Ritchie could be out, judging by Schar’s timeline, Bruce said: “It was nine weeks yesterday, I believe. Fabian is ahead of schedule and he's doing well, but obviously we’ve just got to be a bit careful.
“The kid wants to join in and play, but we’re obviously governed by doctors on that and we’ll see how he is.
“But yes, we're looking at a lengthy lay-off.”
Newcastle’s next match is away at Newport County in the fourth round of the Carabao Cup on Wednesday.