England coach Andy Flower revealed on Saturday that both key batsman Pietersen and wicketkeeper Matt Prior are to have scans on their right knee and left calf respectively, after suffering discomfort in the tour match against Australia A in Hobart.
Additionally, Pietersen - who spent three months out with the same injury earlier this year - is to have a cortisone injection and on Sunday flew not with his team-mates to Sydney for their final warm-up match but for expert medical opinion and treatment in Melbourne.
However, by early afternoon local time he was able to tweet: "I AM FINE", followed by '#relax'.
Pietersen gave no further context to his message for his many followers - but after the bombshell about his injury, only 12 days before the start of the Ashes, it appears to refer to the injury.
His previous problems in the same joint mean there are sure to be heightened concerns about his fitness for the Ashes opener in Brisbane on November 21 - a match which should be the mercurial batsman's 100th Test.
The same injury kept him out of the final Test of England's drawn series in New Zealand in March and subsequently the Indian Premier League, the start of the domestic season, two Tests, three one-day internationals against the Kiwis at home and then the Champions Trophy.
He returned in the nick of time for the start of last summer's Ashes, which England won 3-0 to put them on course for a fourth successive series win against Australia and Pietersen himself the fifth of his career.
Flower said: "Kevin has had a slight recurrence of that knee issue, and he'll be going to Melbourne for a scan and a cortisone injection tomorrow morning - so he won't be travelling with the rest of the team.
"I don't think it's going to be a huge issue. He's had a couple of these injections before, and they've been successful.
"They've quietened the problem down, and it's obviously an ongoing issue for him.
"They have been successful, and we anticipate this one being successful and anticipate him playing a full part in the Ashes tour."
England still hope, in fact, that Pietersen will be fit for their match against an Australian Invitational XI which starts at the SCG on Wednesday.
There appears to be a similar degree of anxiety over Prior, a pivotal mainstay in England's last three successful Ashes series and Alastair Cook's vice-captain too.
Flower said: "He is a concern. He's tweaked something in his calf.
"We're not sure of the extent of the injury as yet ... he will obviously need a scan."
Meanwhile, Cricket Australia have confirmed that captain Michael Clarke and pacemen Ryan Harris and Peter Siddle will continue their preparations for the first Ashes Test at the high performance facility at Allan Border Field in Brisbane, sitting out the third round of the Sheffield Shield as they work their way back to full fitness.
Australia coach Darren Lehmann said: "We're very pleased with Michael's preparation ahead of the first Test. The plan for his Ashes build-up was to play two Sheffield Shield matches, which he has done successfully.
"He has spent valuable time in the middle batting and fielding. Importantly his back has coped well in both matches, but we will continue to take a sensible and cautious approach to managing him. As part of his preparation for the first Test, he will go to Brisbane for training.
"On the fast bowling front, Ryan and Peter are both in great condition. As part of their management plan they will continue their preparation under the guidance national coaching staff in Brisbane over the coming days."
Australia will name their squad for the opening Test on Tuesday.