Let's break it down, shall we? In 1996, when Rivera was still a setup man for John Wetteland, he went 8-3 with a 2.09 ERA in 107 2/3 innings. He finished third in the NL Cy Young voting. This year, Madson went 4-2 with a 3.05 ERA (almost a full run higher than Rivera's) in roughly 25 less innings. Not exactly Mo-like. And that's not even taking into account the entirety of Mo's Hall of Fame career.
The "debate" basically comes down to this simple question: Who would you rather have coming out of the bullpen with the game on the line? Exactly.
I have nothing against Ryan Madson, per se. He's a solid reliever, like Julian Taverez in his prime. Or Tom Gordon. But he's certainly no Mo. My beef is with Boras, who, at this point, has about as much credibility as an anonymous YouTube commenter or William Kristol.