Vice-captain Courtney Lawes and hooker George McGuigan have been forced to withdraw from England's Six Nations squad due to injury, the Rugby Football Union (RFU) announced on Monday.
Lawes withdrew due to a calf injury after he limped off during Northampton Saints' clash with La Rochelle on Saturday while McGuigan injured his knee.
"Tom Dunn and David Ribbans have been called up to the squad by head coach Steve Borthwick," the RFU said in a statement.
England begin their Six Nations campaign against Scotland at Twickenham Stadium on Feb. 4.
The injuries give new coach Borthwick a selection headache with fellow hooker Luke Cowan-Dickie set to miss the start of the Six Nations due to an ankle injury that requires surgery.
Jamie George suffered a concussion when Saracens played Edinburgh in the Champions Cup on Sunday.
George sustained the injury in a tackle and could be stood down for 12 days per World Rugby protocol, meaning he may not train until the day before the opener against Scotland.