I doubt Manny has that much guile in him to be sending subliminal shots through Roger's old nickname.Yeah, I saw some boxing writer post it may have double meaning. They may have been looking too much into it. Sure it was a black mamba tee from Kobe, but Roger Mayweather's nickname was also black mamba.
Most likely that he is a Kobe fan and that was the Kobe shirt Nike brought to the video shoot that Manny liked the most.