According to Alan Pardew, the former Inter Milan star was "rested from Newcastle’s 1-0 win over Aston Villa on Sunday and is "a doubt" for the next game at Hull City.
The 23-year-old is believed to have had a one-on-one meeting with Pardew following United’s 4-0 loss against Tottenham Hotspur, when he was replaced at half-time.
He is well off form, and he is not the player you can call on when the team is in a crisis.
Santon has two years left on his current contract at St James’ Park and it is thought the club will make him available in the summer.