Operating room spies tell TMZ Thiago Alves just underwent an 8 hour operation to separate an artery from a vein in his brain.
Dana White tells TMZ the operation -- in a New York City hospital -- was a complete success and Pitbull's first words ... "When can I start training again?"
Dana's answer ... a couple of weeks.
We're told Pitbull will be in ICU for the next two days.
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