The midfielder limped out of Sunday’s encounter after a second-half challenge by former team-mate Lee Cattermole.
The ex-Black Cat has already had one scan on his injured knee, but is waiting to see the specialist again later this week.
However, Newcastle fear that Colback is facing months, rather than weeks, on the sidelines, and that they will be without him until early next year.
The 25-year-old left the Stadium of Light on crutches at the weekend, with his knee in a brace.
Assistant manager Paul Simpson said the brace was just precautionary until they see the extent of the damage.
“Jack Colback is seeing specialists this week,” said Simpson. “He left the stadium on crutches and with a knee brace on.
“To be honest, that is more of a precautionary measure, we don’t know the full extent of the damage.
“He has had one scan and has a second (scan) later in the week, just to assess the scan and find out exactly what the damage is.”
Cattermole wasn’t booked for the challenge, and Simpson insists there is no blame attached to the combative Sunderland midfielder, insisting that it was a fair tackle.
“He (Colback) was obviously very sore when it happened and I have to say I didn’t think it was a bad challenge,” added Simpson.
“I don’t lay any blame on Cattermole for the challenge. It was one of those wholehearted tackles that you see in derby games, that we all want to see.
“It’s just unfortunate that Jack took the brunt of it, the way his standing leg was, taking the full force of it.”