Less than five years ago, Cody Garbrandt was the UFC bantamweight champion. Now he's unranked and is 1-5 in his last six fights. Former opponent TJ Dillashaw thinks he knows why Garbrandt tumbled out of contention so quickly.

"He's not the smartest human being inside that cage," Dillashaw said during a recent appearance on "The Schaub Show."

He added: "To stay on top, you have to change who you are. And fight to fight to fight, he's the exact same fighter."

Dillashaw and Garbrandt trained together for years at Team Alpha Male before becoming rivals after Dillashaw left the gym.

Dillashaw challenged Garbrandt for the 135-pound belt in 2017, winning by second-round knockout. He also won an immediate rematch in 2018.

From there, Garbrandt lost three of four fights - two by knockout and one in a lopsided decision. His only victory since dethroning Dominick Cruz in 2016 came against Raphael Assuncao.

While Dillashaw considers Garbrandt "a pretty freak athlete," he said the 30-year-old needs to move away from his aggressive, stand-up style of fighting - because it's not working.

But whether Garbrandt will actually make that adjustment is another question.

"He knows how to wrestle," Dillashaw said. "Why not learn some f------ jiu-jitsu and stop relying on your chin and hands, because your chin's not there. ... He's just not willing to do that."