McCarthy landed awkwardly in a marking contest in the last quarter of his side's 10-point loss to South Australia, and was helped from the field.
There were initial fears he had hurt his ankle but the West Australian camp believe McCarthy has broken his fibula and will miss the rest of the season.
State talent manager Raff Guadagnino said he hoped McCarthy's long-term injury would not hinder his draft prospects.
"He's very Nat Fyfe-like, I've got to say. He's raw, he hasn't played a lot of footy, he comes from a cricket background," Guadagnino said.
"But just the way he's able to read the ball in flight, it's probably the best I've ever seen."
The agile tall forward kicked two goals against South Australia before the injury, on the back of his after-the-siren goal last week which handed WA a win over Vic Country.
Recruiters this week believed he had pushed himself into top-20 draft calculations.
Guadagnino said McCarthy was remaining positive but was due for an extended stay in Melbourne before heading back to Western Australia.
"If you know Cam, he's very cool, composed and gets on with it. He keeps things in perspective and I think he even had a bit of a joke (in the rooms)," he said.
McCarthy's injury comes on the back of a likely season-ending collarbone break to WA captain Dom Sheed last week, with injuries the main sour note in what has been a strong carnival for the state.
Next year also looks positive, particularly with Clem Smith and Jarrod Pickett already impressing.
Both are eligible for the 2014 NAB AFL Draft, and Guadagnino lavished praise on half-back flanker/midfielder Smith.
"I think Smith is the best thing I have seen in a long, long time," he said.
Pressed on where he sat with prodigy Jack Martin, who was mini-drafted to Gold Coast last year, Guadagnino said he was more consistent.
"'Clem' gives me four quarters. Jack Martin, at his peak during the under-16s and under-18s, if you go through the tapes, he did some really special things," he said.
"But a player who plays four quarters of footy will always get a tick in my book."
Callum Twomey is a reporter for the AFL website. Follow him on Twitter: @AFL_CalTwomey.
