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.."