Earl could be out of action for as long as three months with a right knee ligament concern, picked up in the 38-10 Gallagher Premiership triumph at The Stoop on Saturday.
Earl ousted Saracens team-mate Billy Vunipola, producing a string of hugely impressive performances as England finished third.
The back-row forward left The Stoop in a knee brace and on crutches as a precautionary measure on Saturday.
But further examinations are understood to have Saracens concerned that he could be sidelined for as long as 12 weeks.
England will start their Six Nations campaign by facing Italy in Rome on February 3, which could leave Earl facing an uphill battle to be ready for Test match action.
Saracens star Vunipola will be first in line to feature at No8 for England should Earl not be ready to start the Six Nations.
Quins’ Alex Dombrandt will also come into the frame, with new Saracens recruit Tom Willis and Bath’s Alfie Barbeary further contenders.
Saracens were wary of committing to a timeline for Earl’s injury in the immediate aftermath of Saturday’s six-try thrashing of Quins, but rugby director Mark McCall did admit he could be facing a spell out of action.
Saracens and England hooker Jamie George admitted the north Londoners still have hopes that Earl can be back on the field sooner rather than later.
“He’s in the form of his life, so fingers crossed it’s not too severe,” said George.
“He just said he felt something in his knee in the warm-up, and it seemed pretty freakish.”
England boss Borthwick took Vunipola to France intent on driving the powerful No8 back to full sharpness and top form.
But Vunipola struggled to move past a knee injury that ended last season prematurely, then the 31-year-old also missed the World Cup opener against Argentina through suspension.
England will look to balance starting a new cycle and benefiting from their vastly experienced stars in 2024, so Borthwick may still be keen to lean on the likes of Vunipola for at least the medium-term.