England could be without injured prop Mako Vunipola for the Six Nations, according to Saracens director of rugby Mark McCall.
Vunipola suffered a knee injury last Sunday to join brother and fellow England international Billy Vunipola on the sidelines.
Speaking after his team's 21-6 Premiership win over Newcastle on Saturday, McCall indicated the suspected ligament damage sustained by the 25-year-old could yet rule him out of selection contention for the tournament.
"It's not looking good [for Mako] to be honest," McCall said when asked about his potential involvement in the Six Nations.
"He's got to go and see his consultant in four weeks' time, and then we will determine how long he will actually be out.
"We have to let the injury settle down as much as possible to have any idea because otherwise you are just guessing.
"It's a big blow to lose Billy and then Mako. Both are such important players or us. We were going to lose them for the Six Nations but to lose them in the run-up to that is so disappointing."
England begin the defence of their Six Nations crown at home to France on February 4.