"The broken rib actually made a small puncture in his lung," said the team's chief medical officer Max Testa in a statement. "The first priority is the spleen injury - that is considered a medical emergency."
The doctor said a decision on when and where to operate on the rider's leg would be made later.
Ballan was 2008 world road champion and finished third in this year's Paris-Roubaix classic.
He also won the one-day Giro della Toscana in Italy and had been due to compete in the season-opening Tour Down Under in Australia next month.
"His loss means a lot since Alessandro is one of our key riders for the classics," said BMC team sporting director John Lelangue. "But most important right now is that he can recover and just get back to his normal process.
"We'll wait for all the medical information before even beginning to think about what could be his plan for coming back."
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- medical emergency