England flanker Tom Johnson is out of the entire Six Nations Championship after a knee ligament injury, while trio Alex Corbisiero, Freddie Burns and Manu Tuilagi are all likely to miss the opening match against Scotland at Twickenham.
Johnson has suffered a torn medial ligament to the left knee and has withdrawn from the squad, ruling him out of the whole Six Nations campaign.
Corbisiero, who was absent from the autumn series last year, and Burns will miss the Calcutta Cup match against Scotland, but Tuilagi remains a doubt with an ankle injury.
"We've had a good week in Leeds so far," Lancaster said. "We're delighted that Saracens and Leicester won their European games on Sunday, but we didn't ask the players from those clubs to train yesterday.
"We had about 20 training on the rubber crumb at West Park Leeds yesterday and we have about 30 training today, including the Saracens and Leicester boys.
"Alex Corbisiero's knee is not settling down, there is a loose element in there so he is going to get that cleaned out today. The important thing for him is to get it looked at, get some game time for London Irish and see how it goes from there.
"Tom Johnson has unfortunately suffered a torn medial ligament in his left knee which expected to keep him out of the Six Nations. He'll stay with his club Exeter Chiefs to continue his rehabilitation and keep in touch with the England medical team.
"Freddie Burns' knee injury is not as bad as first feared but it is also an MCL injury. It will keep him out of the equation for the Scotland game.
"Manu Tuilagi is making good progress with his ankle injury but we are only 10 days away from the Scotland game and can't make a decision until nearer the time. Billy Vunipola has also sat out training this week with an ankle injury.
"Alex Goode has trained this week after his shoulder injury and we're going to send him back to Saracens for some game time in the LV= Cup at the weekend."
Lancaster's men begin their campaign against Scotland, before a trip Ireland, home matches against France and Italy and a final weekend visit to holders Wales.