Northampton duo Lawes and Hartley are to undergo scans as England coach Stuart Lancaster waits to discover if they will miss any of the autumn Tests.
The Saints pair picked up their injuries in Saturday's Aviva Premiership defeat to Saracens and are doubtful for at least England's opening QBE International against Fiji on November 10, which is followed by Tests against Tri-Nations powerhouses Australia, South Africa and New Zealand.
Harlequins lock George Robson was added to the England squad as cover for Lawes on Sunday night, with London Irish hooker David Paice providing back-up for Hartley.
Head coach Lancaster has also called up Worcester prop Matt Mullan as Quins loosehead Joe Marler nurses a tight hamstring.
The England boss will be concerned at the prospect of starting the autumn without two of his first-choice pack, particularly as Lawes missed a large part of last season with a left knee problem, although it is his right that is troubling him on this occasion.
Lawes and Hartley will have their scans on Monday evening, with the results being made public when Lancaster speaks to the media on Tuesday evening.
An RFU statement read: "Courtney Lawes and Dylan Hartley of Northampton Saints will this evening have scans on knee injuries and a further update on their progress will be given at Stuart Lancaster's 5pm briefing tomorrow (Tuesday)."