Shelvey was this week handed the suspension after being found guilty of racially abusing a Wolverhampton Wanderers player by an independent panel.
The midfielder – who denied the Football Association charge – could yet appeal the decision and a £100,000 fine.
Shelvey and the Championship leaders are awaiting the written reasons from the panel for the verdict.
If the 24-year-old appeals, he will be able to play in the Boxing Day home game against Sheffield Wednesday.
Colback, meanwhile, hasn’t played since the November 20 game against Leeds United at Elland Road.
The midfielder – who suffered a gash above his eye in October’s EFL Cup win over Preston North End – has not been able to join in full training because of a vision problem.
United manager Rafa Benitez has repeatedly stressed that the club will not take any risks with the 27-year-old, whose wound needed several stitches.
Colback, a candidate to replace Shelvey, was taken to hospital after Preston’s Alan Browne recklessly caught him with his forearm in the fourth-round tie at St James’s Park.
Browne was sent off for the challenge on Colback, who has been training on his own as a precaution.
Benitez also has midfielder Cheick Tiote, likely to leave the club in next month’s transfer window, available to deputise for Shelvey.
Long-serving Tiote has only made one substitute’s appearance so far this season.
The 30-year-old – who has come close to leaving United in the last two transfer windows – is in the final year of his contract at St James’s Park.