Londoner Haye, the WBA heavyweight champion, finally agreed a deal to fight IBF/WBO king Wladimir over the weekend.
However, the Klitschkos' manager Bernd Boente has revealed that the contract signed does not specify which of the Ukrainian siblings he will meet.
Should Wladimir fail to recover from an abdominal injury before the end of June, Haye could instead fight his brother Vitali, the WBC champion.
Boente - who also confirmed they are exploring possible venues outside Germany for the fight on June 25 or July 2 - said: "It could also be Vitali.
"In the talks, David was not rejecting Vitali any more, he was very flexible. That was never negotiated.
"Knock on wood it's not the case, but should anything happen injury-wise (to Wladimir), as long as it is not three days before the fight or whatever, then Vitali could be the opponent. But first of all Vitali has to win his fight against Odlanier Solis on March 19 which will be a pretty tough fight."