The singer — whose sixth album was called Intensive Care — was showing off when he hurt himself leaping from the top of his recording studio.
His injury couldn't come at a worst time as the Angels singer wants to get back in shape ahead of the May 27 Soccer Aid charity tournament.
The former Take That member — who had a big hit with Back For Good — is due to captain an England squad against the Rest Of The World team, led by actor Michael Sheen.
But Robbie is terrified he has jeopardised his chances for the match at Old Trafford in Macnchester after his attempt to prove his athleticism went wrong.
He wrote on his blog: "I'm watching you Michael Sheen! Your next role better be 'My Left Foot Two' 'cos that's all you'll be able to play after I've finished with you.
"Having said that, I've got to fix my back.
"I jumped off the studio roof a week ago 'to see if I still could' - evidently I can't...
"Lesson learnt; when you stop jumping over things to see if you can, leave it."
Robbie initiated the Soccer Aid charity reality show, which raises money for UNICEF, back in 2006 with television presenter Jonathan Wilkes.
The Rudebox star recently set up a poker website to play online with fans.
Robbie admits the game is beginning to "take over" his life, but is enjoying bonding with his admirers.
He wrote on his blog: "FunPoker is mental! Amazing banter, genuinely laughing out loud with you all, you funny f***ers.
"It's taken over my life right now.
"Thank God I'm not recording at the moment, I'd get nothing done.
"I always wanted a game to play with you all. I suggested backgammon a while ago but this is perfect."
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