It never rains but pours for Six Nations strugglers, and a deluge duly dropped on Scotland during yesterday morning's training session at Murrayfield when Glasgow centre Alex Dunbar suffered a potentially serious knee injury and was immediately ruled out of tomorrow's Calcutta Cup clash with England at Twickenham.

The 24-year-old was taken to hospital for scans, which confirmed initial suspicions that he had damaged his anterior cruciate ligament. Dunbar will now undergo surgery, and the expected recovery time of six to nine months means he will miss the Rugby World Cup later this year as well.

"We're a team who don't get things easy," sighed Scotland head coach Vern Cotter. "This is another example of it. But I think it has tightened the group up, and that will be important when we play this weekend."

There was no contact involved in Dunbar's injury, which happened as he was changing direction during some attack drills. "We'll miss him in attack and defence," Cotter continued. "He's one of our best line breakers and has been playing very well up to now, very well.

"He is with some competent surgeons at the moment. He's understandably very upset about it."

Dunbar's place in the side goes to Matt Scott, the 24-year-old Edinburgh midfielder who has played 22 times for Scotland since making his debut three years ago. Scott has suffered a number of injury problems over the past year, but he was back to his best in his club's Guinness PRO12 victory against Treviso last weekend.

While Scott is a strong replacement, his promotion from the bench leaves Scotland short of cover in the centre. Boldly, Cotter has decided to go with a 6/2 split between forwards and backs among the replacements, a move that could, in certain circumstances, see Johnnie Beattie, the No.8 against Italy two weeks ago, deployed in the midfield.

Cotter has made four other changes to the side he sent out against the Italians. Among the forwards, David Denton has been giventhe No.8 slot Beattie has vacated, while Jim Hamilton takes over from Tim Swinson in the second row. Behind the scrum, Dougie Fife replaces the injured Sean Lamont on the right wing and Finn Russell returns at fly-half after serving the two-week ban he was given for his clash with Dan Biggar against Wales.

While Cotter favours the sort of fast-moving game that is not exactly Hamilton's most obvious strength, he said that the Saracens lock had strengths that were particularly well-suited for the Twickenham match.

"England are a big physical team and they play a very percentage-orientated game, based on occupation, field position and a strong pack," said the coach. "We are looking at trying to compete and put them under pressure in what they consider their strong points. If we can contest their keep the game as tight as possible for as long as possible then you never know.

"Jim is there for his experience. He knows their players very well and I think he may be able to unsettle them with his individual knowledge of them. He is there to give us that solid base at scrum and lineout."

While Sean Maitland had been expected to return to the side, Cotter revealed that the winger had been ruled out early in the week as the shoulder injure he has been carrying is not yet fully healed. However, he stressed that he was happy with Fife in the wing role.

Cotter said: "Dougie's energy, enthusiasm and his kick-chase game are good. He is a young player who is strong, creates line breaks and is a finisher as well."