Roberto Martínez has instigated a revamp of Everton’s medical department to try to resolve James McCarthy’s persistent injury problems and the remarkable number of hamstring strains suffered on his watch as manager.

Everton have sustained 20 hamstring injuries to 11 different players since Martínez became manager, with Leighton Baines the latest victim ahead of Tuesday’s FA Cup tie against West Ham United. The England full-back is available for Manchester City’s visit to Goodison Park on Saturday but the influential McCarthy, who has featured only twice since 27 November due to a third hamstring problem of the season, will again miss out.

Danny Donachie, Everton’s long-serving head of medicinal services, recently left the club and has been replaced by the physiotherapist Matt Connery. Martínez denies Donachie departed as a consequence of tensions but the manager is looking to appoint new staff to the medical department.

“It worries me the number of players who have got re-injured and that’s something we need to address,” the Everton manager said. “We need to get down to the detail of why that is happening. When you injure the right hamstring you’ve got a bigger chance of injuring the left hamstring and that pattern is worrying.”

McCarthy has been a major loss for a struggling Everton side who have won only one league game without the Republic of Ireland international in the starting lineup this season, last month’s 3-1 defeat of Queens Park Rangers.

The 24-year-old had suffered hamstring trouble prior to being played on with an injury against Wolfsburg in November but Martínez claims the midfielder’s physical development is the root cause.

He added: “I’m not going to take a risk [against City] because he’s 24 and still growing and I think this season is a time when his tissues need real care. He played 38 games with me in one Premier League season so he’s someone who’s never had problems with games but this season we need to make sure we get him back to being fully fit and not taking any risks.

“There have been two big changes with James this season, one is the amount of international football he’s played, and that has brought extra work [McCarthy has played once for Ireland this season, against Georgia in September].

“The other is he is still growing. That means the tissues are more sensitive. It is important we find a way to help James in the future.”

Donachie, son of the former Everton assistant manager and Manchester City full-back Willie, had worked for the club for over a decade before his sudden departure last month. “Danny had been here for a long, long time and he felt it was time for him to move on,” Martínez explained. “I wouldn’t say it’s the case he moved on because we had disagreements, not really.

“It’s like anything in these situations – you need to find answers, you need to work hard, it maybe becomes harder to work every day than others, but I don’t think it had anything to do with that. Danny resigned and Matt Connery, who’s been here for 12 years, is taking over but we’re trying to bring more people on board.”