"It was a very dark day when Strikeforce was sold," said Shamrock on The MMA Hour.
In the early days of the UFC, Shamrock held the company's light heavyweight championship and in Strikeforce was the middleweight champion. In both companies, he was the first to hold said championships.
"I think it's because we had fought so hard against the unbeatable adversary the UFC," said Shamrock. "I had so much personally invested in the vision, dream or chance of Strikeforce. It was my whole life. That's all I did."
After going 1-2 under the Strikeforce name, Shamrock retired in 2010 following back-to-back losses.
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