Robert Irwin has been killing it on "Dancing with the Stars" ... but now his fate is in jeopardy, as his pro partner Witney Carson revealed he's suffered an injury ahead of tonight's finale!

Witney posted a video on TikTok Monday, saying ... "Today at [camera] blocking is a little stressful because we just have to make the freestyle all come together. Robert’s ribs have been killing him, and more will be revealed on Tuesday, and you guys can get a better idea of what we’ve been dealing with all week.” Getty

Clearly, the 11 weeks of grueling dance rehearsals have taken a toll ... but despite this, Witney said Robert is "going to want to practice until the very last second."

Gotta love his heart ... plus, if he raises the Mirrorball trophy Tuesday, that would make 2 for his family

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