He advised the club to pay the Germany International whatever he wants to avoid letting him leave the club.
The 28-year-old has entered the final year of his contract and is yet to agree a new deal with the Blues.
Bayern Munich have already shown interest in signing him and are ready to make him an offer that would be a huge rise on his current wages.
But Petit, a former Chelsea defender, reckons the club cannot afford to let him leave on a free in the summer.
“If he leaves, it would be a real pity for Chelsea,” Petit told bookmakers.co.uk.
“He could leave on a free and Chelsea would have to spend a minimum of £30m to find a new defender of his level, and there’s never a guarantee a buy will work out.