Colback left the Stadium of Light on crutches after Sunday’s 3-0 defeat to Sunderland.
And the club is sweating on his fitness ahead of next month’s visit to fellow strugglers Bournemouth.
The ex-Sunderland midfielder suffered a leg injury after former team-mate Lee Cattermole clattered into him early in the second half.
And Colback – who was given a torrid time by Sunderland’s fans on his second visit to the Stadium of Light as a Newcastle player – yesterday had a scan.
The club’s medical staff were today analysing the results.
But it is feared the 26-year-old – who is suspended for Saturday’s home game against Stoke City after picking up his fifth booking of the season – will be sidelined for a number of weeks.
Cattermole was not booked for his challenge on Colback – or several other incidents.
And United felt combative Cattermole – who caught Georginio Wijnaldum in the home box seconds before Sunderland broke upfield an won the penalty converted by Adam Johnson – was fortunate not to be shown a yellow card by referee Robert Madley.
Madley booked Newcastle’s Cheick Tiote early in the match, and the midfielder had to be substituted at the break.
Head coach McClaren refused to talk about Cattermole – who he once managed at Middlesbrough – after the match.
Asked if he felt the 27-year-old should have been booked, McClaren said: “I’m going on and on about this referee making wrong decisions, aren’t I?”