The striker had to have a three-hour operation in Glasgow recently to remove five cysts from his jaw which were caused by a mouth infection.
A frustrating season
This news has ended what has been the most frustrating season in McCormack's career. He was signed by Aston Villa for £12.5 million from Fulham just last summer, and he was seen as a key signing in Villa's quest to get back to the Premier League at the first attempt.
These ambitions have not worked out for Villa this term, as they find themselves sat in mid-table despite improving under Steve Bruce throughout the last few months.
Not only has this season not gone to plan for Villa, but also McCormack himself. He only scored three goals for the club before a fall-out with Bruce over the 'gate' incident resulting in the Scotsman turning up late to training. It is unlikely that Bruce will have plans to use him next season.
He did manage to score on his debut for Forest after signing on loan for the remainder of the season on Deadline Day. He scored Forest's only goal in a 5-1 defeat to Norwich City at Carrow Road but has not featured for the Reds since the 2-2 draw against local rivals Derby County at The City Ground on 18th March.
Can he get back to his best?
Before this season, McCormack was known throughout the division as being one of the best finishers and his track record supports this. He scored 23 goals for an average Fulham side last season which attracted Villa to make the move.
He scored 38 goals in 89 appearances for Fulham in total and also had a successful spell at Elland Road with Leeds United where he scored 53 goals in 144 appearances.
His future with Villa remains unclear, but it is likely that Bruce will look to move him on in the summer transfer window. There are still going to be a number of Championship teams willing to take a chance on him considering his history of being able to find the back of the net, if he is able to prove his fitness.
Blow to Forest's survival bid
This news does come as a huge blow to Mark Warburton's side and their chances of survival. At a time when Forest's lack of cutting edge in front of goal has never been more evident, McCormack's quality would have come in use.
18-year-old striker Ben Brereton, Britt Assombalonga and Apostolos Vellios are the only strikers currently being used by Warburton and none of them are currently scoring the goals required to keep Forest in the division.
Forest face a huge clash against Reading at The City Ground on Saturday and will have to find a way of finding the back of the net quickly.