The 19-year-old, who was on loan from QPR, will be sidelined for six to nine months after suffering the injury on his O's debut, the 5-2 FA Cup win over Southport on Saturday.
Rangers coach Marc Bircham, who was at Brisbane Road to watch the youngster, told the club website: " He went in for a slide tackle and got whacked on the back of the calf, I knew he was hurt because he stayed down and that's not like him.
"He was playing well, and I know the guys at Leyton Orient were over the moon with him.
"It's a setback for Frankie who is one of our brightest young hopes. Hopefully he can return from this fitter and stronger than ever. He's only 19 so he's young enough to do that.
"We have got a good medical team here and they will help him through. One thing Frankie won't be lacking in his recovery is determination because he is a dedicated lad who loves his football.
"He's got a good family around him who will support him, and we are all here to support him as well."
- Sports & Recreation
- Leyton Orient