The Holland international was hurt by a Jonathan Cristaldo challenge and hobbled away from the Metalist Stadium on crutches following Thursday night's Europa League victory over Metalist Kharkiv.
The 1-0 win secured the Premier League club's safe passage into the last 16.
Newcastle confirmed the 24-year-old underwent a scan on Friday afternoon and will face further tests at the start of next week.
Krul tweeted on Friday night: "Gutted about my ankle after a fantastic win last night..Thanks for the nice messages.. Hope to be back playing soon.."
A club statement read: "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.
"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."
It will be a big blow for Newcastle boss Alan Pardew as he prepares for the visit of his former club, Southampton.
Krul later said on Twitter: "Gutted about my ankle after a fantastic win last night. Thanks for the nice messages. Hope to be back playing soon."