The Gloucester number eight was injured in England's win against Scotland on the opening weekend of the Six Nations, but is expected to return against Italy on March 10.
"Ben will be fine. It's an ankle injury similar to the one Manu Tuilagi had. Manu was out for four or five weeks and Ben will be about the same. He wants to get himself right so he can push for selection against Italy," Lancaster told reporters.
However, fly-half Toby Flood missed training on Monday with a tight calf.