McCartin will be out of action for at least this Saturday's clash against Port Adelaide after suffering a head knock in the club's two-point win over St Kilda on Sunday.
The 26-year-old had gone back with the flight of the ball in the third term and Saints small forward Liam Ryan cannoned into his back.
McCartin missed two games last season due to concussion, suffering a head knock in the club's round-18 win over St Kilda.
He missed a total of 15 weeks in the previous three seasons, first missing nine weeks in 2023, before spending four weeks on the sidelines in 2024.
McCartin's brother Paddy was forced to retire from the AFL two years ago because of concussion issues, while West Coast great Jeremy McGovern was medically retired by the league's concussion panel last year.
Coach Cox was certain of McCartin's return this season, but put no pressure on a swift comeback.
McCartin had been in All-Australian form, boasting an "elite" average of 20.1 disposals and three intercept marks to help second-placed Sydney (11-2) establish themselves as flag contenders.
"He's going well. He's been in the footy club all the way through the week. He's healthy," Cox said on Thursday.
"He's responded well. He's just now working through some protocols, and we'll hopefully build towards some training next week and assess him from there."
Asked if the club was also managing the emotional aspect of McCartin's latest concussion, Cox said: "No, not really.
"He's had a few concussions like most players, but he's vibrant when he comes back into the football club.
"He's focused on trying to get back as quickly and as safely as he possibly can, and we'll give him that time."
Cox said Justin McInerney was "flat" after being sidelined for at least eight weeks following a seemingly innocuous hamstring issue.
Excelling in injured Errol Gulden's absence, McInerney was forced out of the St Kilda clash late in the first quarter with hamstring soreness.
Scans later revealed a high-grade tear, with McInerney - at worst - facing up to 10 weeks out of action.
The 25-year-old arrived at Thursday's main training session on a scooter.
"He's OK. He's flat, obviously, with, probably, the news being a little bit worse than what he thought throughout the game," Cox said.
"Like all good leaders do and professional athletes, he'll prepare as well as he can.
"He'll be around the boys as much as he possibly can to make sure that we're still performing at the level we need to."
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