Good news, Cardinals fans ... Nelly's gonna get Bryce Harper to join the squad!!!
At least, that's what the rapper says -- 'cause when he grabbed baseball's top free agent backstage at the Florida Georgia Line concert Saturday ... he gave his best St. Louis pitch.
"Listen, Bill, Mr. DeWitt, call me! I got this. Alright? I'm doing all the negotiations. He just need a little convincing, man! That's all!"
Bryce's response -- he didn't say no!!
Of course ... Bryce has a TON of suitors these days -- even ones just as famous as Nelly .