Chelsea will be without Michael Essien for the majority of the forthcoming season after the Ghana midfielder had surgery to repair a ruptured anterior cruciate ligament and meniscus, an injury that will rule him out until well into the new year.

Essien twisted as he landed awkwardly last Thursday – the first day of double sessions since the first-team squad returned to Cobham to begin preparations under André Villas-Boas – and, once the swelling on his right knee had receded slightly, underwent a series of scans. Those confirmed Chelsea's worst fears, with the injury a repeat of that suffered three years ago, and necessitated the 28-year-old having surgery on Monday.

That operation was described as "successful" by the club, though the midfielder will not be able to return to training for at least six months and, given his history of knee injuries, there is a sense his recovery could drag well into next spring.

"Everyone at Chelsea wishes Michael well for his recovery," said Villas-Boas. "His team-mates and all the staff and I look forward to helping him return to playing as soon as possible."

The Ghanaian managed 43 appearances last term – albeit rarely displaying his previous dynamism – which was considered encouraging given he suffered serious injuries to both knees in recent seasons. He had been ruled out for more than six months from September 2008 after rupturing the same anterior cruciate ligament while playing for Ghana against Libya, and missed seven months last year after tearing the medial ligament in his left knee while training ahead of the African Cup of Nations.

That injury kept him out of the summer's World Cup finals and eventually required four operations.Given those previous problems, there will inevitably be concerns over whether Essien will ever return to being the explosive, energetic player who cost Chelsea some £24.4m when signing from Lyon in 2005.Another lengthy period of rehabilitation now awaits before the player can consider any return to competitive action.

The midfielder's prolonged absence could yet force Villas-Boas to enter the transfer market in search of a direct replacement, despite the presence of the Brazil international Ramires – who is absent at the Copa América – and Mikel John Obi at the club. João Moutinho, a key player at the manager's former club Porto, could be pursued though Chelsea would need to activate a €40m (£35m) release clause in the 24-year-old's contract at the Europa League winners. Scott Parker, a former Chelsea player who was restored to the England set-up last season, is more readily available at West Ham United.

Chelsea are set to make an improved offer this week for the Tottenham Hotspur midfielder Luka Modric, the subject of a failed £22m bid last month. Modric, who trained at Chigwellon Monday, had been outspoken in his criticism of the Tottenham chairman, Daniel Levy, in an interview in the Croatian press at the weekend,though the club's resolve to keep him is unshaken. "At the end of the day, it's up to the chairman," said the Spurs manager, Harry Redknapp. "He makes those decisions. I certainly wouldn't want to lose Luka. But it's a difficult situation. He's a fantastic boy, a great player, and it's not easy when people come along and double, treble your wages. It makes it difficult for him. I understand where he's coming from a little bit with it, but I'm desperate to keep him because, as I say, he's so good. He's not a minute's problem. He works hard, trains hard every day, and we desperately need to keep him if we're going to push on. We need to add to him or we'll fall behind. That's obvious. Manchester City are improving, Chelsea, Manchester United, Liverpool, Arsenal … we need to improve, not weaken the team."