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.
Peterborough United manager Grant McCann has backed Brad Inman to recover from the broken leg he suffered with "six seconds left of training".