Dylan Hartley and Elliot Daly have emerged as doubts for England's clash with Ireland just hours after being cleared to start the final round of the Six Nations.
Defence coach Paul Gustard insisted at lunchtime that the pair had recovered from their respective calf and foot problems and were available for selection.
But following training on Wednesday afternoon, their availability was placed in jeopardy by medical checks, the Rugby Football Union said.
England have delayed their team announcement until Thursday lunchtime to give the duo as much time as possible to recover from their injuries.
Earlier on Wednesday, Gustard said: "Dylan and Elliot were part of training today. We were monitoring them for the first couple of days, but took part today. They're available for selection."
Courtney Lawes and Nathan Hughes, meanwhile, are definitely out, with the back-five forwards scheduled to undergo surgery to repair ligament damage this week.
Hughes re-injured the same knee that was hurt earlier in the season, forcing him out for eight weeks in a spell that encompassed the opening two rounds of the Six Nations, after Joe Launchbury and Francois Trinh-Duc fell into his left leg at the Stade de France.
Lawes managed to finish the game that ceded the title to Ireland with a round to spare despite his own knee complaint, but he has also been ruled out for 14 weeks - a fortnight longer than the rehabilitation period predicted for Hughes.
The time frame for their recoveries all but ends their hopes of being involved in the three Tests scheduled against the Springboks in June.
"We're really disappointed. We wish them the best for their recovery, we've got close links with the players individually and we'll be in touch with their physios so we're part of their rehab process," Gustard said.
"It's an opportunity for somebody else. People heal at different rates so we'll assess their availability for South Africa as the injuries heal I guess."
Eddie Jones is planning at least three changes when he names his team on Thursday.
A return for Hartley would see Jamie George drop to the bench and Owen Farrell relieved of the captaincy he held against France.
With Lawes and Hughes in the treatment room, James Haskell and Sam Simmonds are likely to be promoted from the bench into the back row alongside Chris Robshaw.
Prop Dan Cole and fly-half George Ford are also under pressure for their places in the team following a string of below-par displays in this Six Nations.