The Netherlands international picked up the ankle injury in Thursday’s Europa League last-32 second-leg victory over Metalist Kharkiv, but was able to complete 90 minutes.
Krul was initially assessed by the club’s medical staff on Friday and subsequently sent for a scan, but has now been referred to a specialist - raising fears of a lengthy spell out.
The shot-stopper will not miss Sunday's Premier League game at home to Southampton, with Rob Elliot set to step in for the 24-year old.
"Newcastle United can confirm that Tim Krul was assessed by club medical staff on Friday and sent for a scan on Friday afternoon after sustaining an ankle injury in Thursday night's Europa League match against Metalist Kharkiv in the Ukraine," a club statement said.
"Krul is now scheduled to see an ankle specialist on Monday for a further assessment, after which time the club should be able to provide another update."
Newcastle has recently registered wins at Aston Villa and at home to Chelsea in the league but is still only four points off the relegation zone with 12 games left.