Cambridge United defender Jonathon Thorpe will miss the start of the new season after tearing a cartilage in his knee in training.
The 19-year-old needs surgery and could be out for up to two months.
"It's just a freak injury, really. My experience of cartilage tears is they're always pretty innocuous things.
"There was no-one near JT when he did it. He stumbled and felt his knee lock a little bit," United boss Jez George told BBC Radio Cambridgeshire.
Thorpe missed Wednesday's friendly against Ipswich, which the Championship side won 2-0.
Cambridge included two trialists in their line-up in former Hartlepool full-back Joe Tair and ex-Lindoln forward Nialle Rodney.