England star Vunipola breaks down again
Vunipola received lengthy treatment following the first half injury before getting to his feet and hobbling around the perimeter of the ground aided by two medics.
"It didn't look great and I hope it's not as bad as it looked but he's gone for a scan," Saracens director of rugby Mark McCall said.
"It's the same knee he injured for England against Argentina last November and it's cruel luck as we obviously feel for him terribly."
Despite Vunipola's absence, Saracens controlled the game with five tries.
Ben Spencer scored two of them, Liam Williams, Nathan Earle and Vincent Koch the others, with Owen Farrell converting all five and adding two penalties.
England prospect Marcus Smith was forced off during the decisive phase of Harlequins' 31-28 defeat at home to Leicester.
England coach Eddie Jones watched from the stands as the 18-year-old Harlequins flyhalf, who he recently described as a potential X-factor for the national team, underlined his talent with another assured performance.
Smith eventually limped off in the 61st minute after appearing to suffer a muscle injury while taking a restart, placing doubt over his involvement in the England training camp which starts on Sunday.
Newcastle shrugged off the jet lag after their trip to the United States last weekend to snatch a rare league victory at the Recreation Ground.
After losing to Saracens in Philadelphia last Sunday, Newcastle raced into a 19-0 lead in the first half with tries by Sonatane Takulua, Rob Vickers and DTH van der Merwe.
Bath came back with 32 unanswered points of their own through tries by Matt Banahan, Semesa Rokoduguni, Charlie Ewels and Jonathan Joseph.
But Newcastle surged back to seize the win with tries from Chris Harris and Mark Wilson.
Agence France-Presse
