New-look Chelsea are quick but vulnerable says Manchester United midfielder Darren Fletcher

Darren Fletcher has seen enough of Chelsea over the last few years to know that they have changed radically. The team laden with playmakers under Roberto Di Matteo is a world away from the powerful, efficient machine Jose Mourinho once used to steamroller opponents.

But that does not make them any less dangerous to his club, Manchester United, who trail the Premier League leaders by four points as they prepare to meet them at Stamford Bridge on Sunday.

The speed with which young and quick footballers such as Oscar and Eden Hazard have fitted into the supposedly ageing team that Andre Villas-Boas failed to break up has alarmed their rivals at Old Trafford.

"It used to be a big, strong, physical side," said Fletcher. "You found yourself playing against Ballack, Makélelé and Lampard [who is likely to miss Sunday's game with a calf injury] in midfield and Drogba up front. Now you tend to be facing more Barcelona-style players – the small, intricate ones who get in the pockets behind you and play the killer pass."

Nowhere was that better demonstrated than in the 4-2 victory over Villas-Boas' Tottenham last Saturday. It was the fifth time this season Di Matteo's side had scored four goals or more – in their pomp under Mourinho, the stock scoreline was a 1-0 win.

"Because they are looking to get into the pockets behind you, you are not really up against anyone," Fletcher said. "You have to be aware, look over your shoulder and communicate, because they will want to get in behind you.

"But at the same time, it gives you opportunities to break at them. If we do win the ball, it will leave a lot more spaces than you normally expect against Chelsea. Traditionally, they are very solid, have men behind the ball and are hard to beat. However, with all those attacking players, they may give us spaces to exploit. The neutrals will want a classic but we just want a victory."

Until illness struck him down, Fletcher was one of the men Sir Alex Ferguson would turn to for the kind of encounter United will endure at Chelsea. The Scotland captain was not the most eye-catching midfielder in his manager's armoury but he was among the most reliable – and at Stamford Bridge, where United have not won a league fixture in a decade, reliability is not an optional extra.

Well though the 28-year-old performed in Tuesday night's jittery 3-2 win over Braga in the Champions League, it would be a surprise if Fletcher started a fixture that will be one of the pivots of the season. Ulcerative colitis, a chronic bowel disease that drains its victim of weight and energy meant he had started one match in 10 months and in the summer headlines suggested it may even cost Fletcher his career.

"If you told me in June or July that I would be starting Champions League matches now, I would not have believed you," he said. "I always felt I was one who did appreciate the game and how fortunate I was, but after going through something like this you do appreciate it more."