The 24-year-old midfielder sustained the injury just 26 days after joining Posh from League Two side Crewe.
He had an operation on Saturday and is expected to be out until November.
"I haven't seen a grown man cry like that for 15 years so I feel for him, but we'll just have to pull together," McCann told BBC Radio Cambridgeshire.
"There were about six seconds left of training, he planted his foot and broke his leg.
"He's had an operation, but I'm sure he'll come back stronger. The lads and the staff are devastated for him."
McCann also confirmed that Posh had not taken up the option to sign trialist striker Morgan Ferrier, who played in the pre-season friendlies against West Ham and Norwich.

Nottinghamshire Police said it would expand its categories to include misogynistic incidents.
It means abuse or harassment which might not be a crime can be reported to and investigated by the police, and support for the victim put in place.
Nottingham Women's Centre said it hopes it will help give more victims the courage to report incidents.
Chief Constable Sue Fish claimed it will make the county a safer place for women.
"What women face, often on a daily basis, is absolutely unacceptable and can be extremely distressing," she said.
"Nottinghamshire Police is committed to taking misogynistic hate crime seriously and encourages anyone who is affected by it to contact us without hesitation."
Work on the idea first started with the Nottinghamshire Safer for Women Conference last year, co-hosted by the police with the Nottingham Women's Centre.
BBC TV reporter Sarah Teale was harassed in the street while reporting on the conference.
The force defines misogyny hate crime as: "Incidents against women that are motivated by an attitude of a man towards a woman and includes behaviour targeted towards a woman by men simply because they are a woman."
The classification now means people can report incidents which might not be considered to be a crime and the police will investigate.
Nottingham Women's Centre has been helping train call centre, force control staff and officers on the beat to recognise misogynistic hate crime and ways to tackle it.
These officers will also examine if and how a victim can be supported or if anything can be done to help prevent them being targeted again.
Domestic abuse will not be recorded as a misogyny hate crime because it has its own procedure, the force said.
Melanie Jeffs, centre manager at Nottingham Women's Centre, said: "We're pleased to see Nottinghamshire Police recognise the breadth of violence and intimidation that women experience on a daily basis in our communities."
She added: "Recording this as a hate crime will give us a detailed picture of how often, when and where it is happening. It has been very difficult to build that picture before but we will now get detailed data to analyse.
"Showing that the police take it seriously will also give people the confidence to come forward and report offences."
A crime that the victim or any other person perceives to be motivated by hostility or prejudice towards any aspect of a person's identity.
Police forces in England, Wales and Northern Ireland annually monitor five strands of hate crime:
Forces can include their own definition of a hate crime with several recently adding sub cultures.
Peterborough United manager Grant McCann has backed Brad Inman to recover from the broken leg he suffered with "six seconds left of training".
[NEXT_CONCEPT]
Harassment of women is to be recorded as a hate crime in a bid to tackle sexist abuse.