Billy Vunipola provided England with an injury scare as holders Saracens' European Champions Cup quarterfinal hopes were left hanging by a thread.
The Saracens and England number eight went off at half-time during Saracens' 15-15 draw with the Ospreys at Liberty Stadium.
He appeared to suffer a wrist injury in only his second comeback game following three months' sidelined because of knee trouble.
It completed a concerning day for England head coach Eddie Jones after Wasps flanker James Haskell was sent off in his side's defeat against Harlequins, with the Six Nations just three weeks away.
Saracens, though, now realistically need a bonus point victory over Pool 2 opponents Northampton next weekend to have any chance of progressing as a best runner-up after Owen Farrell's five penalties were cancelled out by five from Ospreys playmaker Dan Biggar.
Saracens' European title hat-trick bid will now hinge on results elsewhere even if they claim a five-point maximum against Saints, while Ospreys must beat Clermont in France to have any chance of progressing.
Saracens were forced into a late change when flanker Jackson Wray withdrew from the starting line-up. He was replaced by Calum Clark, with Schalk Burger taking Clark's place on the bench, while Ospreys centre Owen Watkin recovered from injury to partner Ashley Beck in midfield.
Given what was at stake, it came as no surprise how ferocious the early exchanges were, and Saracens wasted no time getting powerhouse runner Vunipola involved with ball in hand.
Vunipola twice ran aggressively into the heart of Ospreys' defence during an opening 10 minutes that Saracens dominated, with a Farrell penalty putting them ahead as their opponents struggled to establish any territorial foothold.
Ospreys scrum-half Rhys Webb, though, rifled a superb long-range touchfinder to put Saracens under pressure for the first time, then Webb's inch-perfect long pass was spilled by number eight Rob McCusker following Beck's clever flick.
Saracens cleared the danger, but Wales star Webb's attacking instinct had surfaced and the double European champions knew they had to quell his threat.
Biggar opened Ospreys' account with a 30th-minute penalty following a Maro Itoje indiscretion, yet Saracens stormed upfield from the restart and Farrell completed his penalty hat-trick through a close-range strike.
But Biggar again cut the gap to three points when he landed a long-range penalty on the stroke of half-time, meaning that Saracens shaded an attritional first period 9-6.
Saracens boss Mark McCall made a double switch for the second-half, sending on Burger for Billy Vunipola, while lock George Kruis replaced Michael Rhodes, who suffered a hamstring injury, and Itoje moved into the back-row.
Biggar hauled Ospreys level with a 43rd-minute penalty, but Farrell then landed another three-pointer that edged Saracens back in front during a game high on physical collisions and defensive prowess, yet offering little in terms of clear-cut attacking opportunities.
Saracens substitute Chris Wyles was sin-binned for dangerous play following a collision with Ospreys full-back Sam Davies 15 minutes from time, handing Ospreys a temporary numerical advantage and increasing pressure on the visitors.
And Biggar kicked an equalising penalty eight minutes from time, setting up a gripping finale that saw Farrell land a 75th minute penalty before Biggar slotted an equalising strike four minutes later.