The Netherlands international was hurt by a Jonathan Cristaldo challenge and hobbled away from the Metalist Stadium on crutches after the 1-0 victory which sent Newcastle through to the last 16.
Newcastle confirmed the 24-year-old underwent a scan and will face further tests at the start of next week.
A short statement on the club's official website, www.nufc.co.uk, 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."
Manager Alan Pardew had initially been hoping Krul would be fit for Sunday's league clash with Southampton, but the announcement suggests the goalkeeper will not be available.