The Bundesliga outfit announced on Tuesday that the Dutchman is not training with them as he is en route to St James' Park for a medical.
A post on Monchengladbach's official Twitter profile read: "Luuk de Jong (did) not train today with the Foals.
"The attacker (is) just on the way to the medical check to Newcastle."
De Jong has been a reported target for Newcastle for some time, and the Premier League club now appear to have struck a deal with Monchengladbach.
The 23-year-old, who moved to Germany from Twente in 2012, has failed to score in 13 Bundesliga appearances this season.
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