To me, Rosario had massive trouble with guys who refused to engage with him, as well as those who go underneath his offense. If Davis and Camacho can arguably beat him, or do so outright, then so can Brown IMO.
If Rosario didn't simply bomb out a fighter, he had trouble sustaining offense over the course of an entire fight - see Ramirez, Davis, Camacho, and Randall for examples. Rosario's speed and athleticism might get him out to an early lead, but I think Brown opens up at some point and retakes the fight by decision or late stoppage.
Agree with the above posts. Brown, when he was focused and motivated, would have outboxed Rosario and possibly hurt him in the process. Brown's jab and movement will give Rosario a lot of problems, much like Davis and Camacho.
Brown is severely underrated, he never gets talked about much. At his sharpest he was really brilliant to watch. He has that style that reminds me of SRR and Gavilan at times. I can see some Ezzard Charles as well.
Personally I think Joe Brown was more like a combination of Roberto Duran, Ray Leonard, Marvin Hagler, Tommy Hearns, and Wilfred Benitez. And Mike Tyson and Roy Jones and King Kong and Godzilla.
You forgot to add Big Foot, the Loch Ness Monster, and Jesus to the list as well. A rare combination indeed.
