Maro Itoje has suffered a broken jaw and could be a doubt for the start of England's Six Nations campaign.
The England and British and Irish Lions lock was injured in a collision with Mike Brown as Saracens lost out 20-19 to Harlequins in Sunday's Aviva Premiership clash at The Stoop.
Saracens have confirmed Itoje's injury as a fractured jaw, and the 23-year-old will now undergo further testing to determine his recovery period.
England launch their Six Nations title defence against Italy in Rome on February 4, leaving Itoje two months to be fit to contest selection for Eddie Jones' side.
"Scans have confirmed that Saracens forward Maro Itoje fractured his jaw in Sunday's round 10 Aviva Premiership clash against Harlequins," read a Saracens statement.
"The 23-year-old England forward left the field in the 48th minute of Saracens' 20-19 defeat at the Twickenham Stoop and was taken to hospital where scans confirmed the fracture.
"Itoje will now see a second specialist later this week to confirm his return to play schedule."
Itoje's injury is another blow to Saracens, who suffered their fourth league defeat of the campaign after going down to fierce rivals Harlequins.
The second row will also miss Saracens' upcoming European Champions Cup clashes, with the double reigning champions next facing French side Clermont in back-to-back fixtures.
Itoje was nominated for World Rugby's world player of the year for 2017, after a stunning calendar year that comprised England's Six Nations win, Saracens' second European triumph and the Lions' drawn series with world champions New Zealand.
Itoje left The Stoop's field clutching his face immediately after his clash with England team-mate Brown in Sunday's encounter, and was sent straight to hospital for treatment.
Now he faces a further wait to discover how long he will be sidelined.