The former Sunderland man was injured in last weekend's Tyne-Wear derby.
The 25-year old sustained the knee injury after a heavy challenge from Lee Cattermole and has undergone initial scans with further testing to take place in the next few days.
The Magpies had hoped Colback would miss only a matter of weeks but there is now the fear that it could stretch into months, depriving them of yet another important player in what has been an injury-hit campaign.
His absence will increase the pressure on Cheick Tiote to step up, after his own injury troubles, and would leave Steve McClaren’s team in a tricky spot should any further fitness issues arise.
Yoan Gouffran was deployed in a central midfield role earlier this season to disastrous effect and Newcastle will hope a similar scenario doesn’t present itself again this campaign.
Colback’s injury will come as a blow to a team that has shown clear signs of improvement in recent weeks and McClaren will be doing his utmost to ensure this setback doesn’t have too disrupting an effect,