Billy Vunipola will miss the rest of England's Six Nations campaign due to an ankle injury.
The No 8 was sent for a scan on Monday after coming off during the first half of his side's Twickenham victory over Ireland on Saturday.
The results confirmed ligament damage to an ankle, and Vunipola will play no part against either Wales or Italy.
It is not known how much more of the season the Saracens back row will miss.
Ben Morgan replaced Vunipola against Ireland and will be promoted to the starting line-up for the visit of Wales on Sunday week.
Either Tom Johnson or Matt Kvesic will take Morgan's place on the bench.
"The encouraging thing is that we have Ben Morgan and other back rows such as Matt Kvesic and Tom Johnson who are all competing hard for places," Lancaster said.
How many wins will England finish the Six Nations with?