The player had fully expected to be able to play for the Magpies this season, as he was assured that he was part of the head coach Bruce's plans beyond the summer.
Colback now faces an uncertain future as the transfer window in England's top two divisions is closed – and is only available to Scotland or League One or lower. A change to the continent remains an option for the midfielder.
The club decided against the appointment of Colback in his 25-member squad. Rolando Aarons, Jamie Sterry, Achraf Lazaar and Henri Saivet also failed.
Colback, who is deeply disappointed that his dream of being crushed by United again, has only one more year left for his contract with Newcastle, which he signed with a free transfer following his resignation from fierce rival Sunderland.
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Likewise, Colback was frozen by the then manager Rafa Benitez two summers ago.
Bruce, a manager who tried to sign the 29-year-old in the summer of 2016, presented him with his professional debut in Sunderland.
Bruce said, "It's the terrible part of the job that I have to tell them that they do not even have a team number, I do not understand that you have to have a (25-man) team when it comes to all of them Problems, but let's go. "
When asked about Colback, he said, "Unfortunately, he's one of those who does not have a squad number."