Eminem made a surprise appearance at the Live ... musician launching into a performance of "Sing for the Moment" to huge cheers from the crowd. The star also performed with the Detroit Symphony ...

"I'm not afraid," Goodell assured him, tripling down on the Eminem references ... and Calvin Johnson to the stager amid rapturous cheers. Watch footage from Em's appearance below.

He makes no effort to conceal that fact on his debut long-player, Cheers. Having repeatedly stressed in interviews that he's not Eminem and that he certainly isn't Fitty, he slams this point home ...