Saracens and England No 8 Billy Vunipola suffered an injury scare after being forced to withdraw from a training session due to a tight hamstring.
The 25-year-old only returned to action against London Irish on Saturday, after three months out with a broken arm, which he suffered against Ospreys in January.
Vunipola pulled up in Saracens training on Wednesday and is now being assessed by the club's' medical staff, but director of rugby Mark McCall is optimistic the injury is relatively minor.
"Nothing happened, he just felt some tightness and awareness of that tightness. The hamstring hasn't pulled or anything," McCall said.
"It was mature of him not to try to plough on in training. If we have to scan it we will. That's likely to happen, I would have thought.
"When you have been out for a very long time, like he has, you try to get some volume into the legs."
Vunipola has played only 249 minutes over the past year due to shoulder, knee and arm injuries and his last international appearance was against Ireland in March 2017.
Saracens full-back Alex Goode has plenty of sympathy for his team-mate.
"We are just very disappointed for him. He's worked incredibly hard to come back," Goode said.
"He's someone we all love to bits at the club. He wears his heart on his sleeve. "Despite the injuries, he's still been a great person around the club, which speaks volumes.
"He's had a very tough time, as we all know, but he hasn't let it affect him around the boys."
Saracens are already assured of a second-place finish in the Aviva Premiership and a home semi-final in the play-offs on May 19, heading into the final round of games this weekend, when they face Gloucester at Allianz Park.
Eddie Jones names his England squad for the three-Test series against South Africa next Thursday and will be anxiously waiting for a medical update on the influential Vunipola.
England can ill-afford any further losses as they contend with an escalating list of absentees.
Dylan Hartley, George Kruis, Nathan Hughes, Courtney Lawes, Jonathan Joseph and Anthony Watson have been ruled out for the rest of the season, while Bath have implored head coach Jones not to pick Sam Underhill as he continues his recovery from a toe injury.