Saracens number eight Billy Vunipola is struggling to be fit in time for the club's European Champions Cup quarter-final against Leinster on April 1.
The Premiership team's director of rugby Mark McCall has said that the dynamic backrow is facing a race against time to be ready for the Aviva Stadium showdown.
The 25-year-old England international missed the Six Nations campaign after fracturing his arm playing for Saracens in January but was expected to return for the club's trip to Dublin next month.
"It's a little bit uncertain, Billy is in a race against time to be ready for the quarter-final," said McCall, a former Ireland international.
"We'll make a call on what the surgeon says. If the arm isn't right to play, then we won't risk him. He's been injured enough; we don't want to re-injure him.
"Billy is very close to coming back but whether he makes the quarter-final, we're not sure yet."
The arm injury was the latest setback in a wretched spell for Vunipola, who also missed last year's British and Lions Tour and England's autumn internationals due to serious shoulder and knee injuries respectively.
Vunipola last played for England in their defeat by Ireland in the final match of the 2017 Six Nations campaign.
National team head coach Eddie Jones will be keen to see the forward return for the three-test tour of South Africa in June having seen them lose their last three Six Nations games on the bounce.