Joe Cokanasiga and Mark Wilson have emerged as doubts for the start of England's World Cup campaign due to knee problems.
Cokanasiga and Wilson have been unable to take part in training at the squad's camp in Miyazaki due to the injuries and, according to Eddie Jones, they are now undergoing a "mini pre-season".
Both players were involved in the final warm-up Test against Italy, Wilson starting at openside flanker and Fijian-born wing Cokanasiga making a second-half appearance from the bench.
But now a question mark hovers over their involvement in the Japan 2019 opener against Tonga in Sapporo on September 22 and potentially the second Pool C clash with the USA four days later.
"When guys get knocks and niggles they go through a rehab process based on a specific injury," said defence coach John Mitchell, who declined to reveal whether the players could recover in time.
England will be expected to sweep Tonga aside, not least after the Pacific Islanders were overwhelmed 92-7 by tournament favourites New Zealand recently.
But the underdogs have a long-standing reputation for their ferocious tackling that could keep the English medics busy in a week's time.
"The Tongans are a very hard-working people. They're obviously very passionate when they do play for Tonga," former New Zealand boss Mitchell said.
"They're representing not only themselves and their family, but also a small island that's produced a lot of very good rugby players. It will be a very special occasion for them. It's something that will lift them."
England enter the ninth instalment of the World Cup as joint second favourites alongside South Africa and hooker Jamie George knows that the culmination of the Jones era has arrived.
"Eddie's not giving us too many messages now we're here because the message has been drilled into us for the last four years," George said.
"When he first came in he said to us that we're here to win the World Cup in four years' time. We're in that place now.
"We're in a good position to go out and do that. We've got an awful lot of hard work to do to get there, so we're not thinking too far ahead."