Chris Willman Music WriterMaking a good argument that rehearsals are for rookies, and that some Oscar nominees are better utility players than others, Questlove sat in with John Mayer’s band at a Madison Square Garden show Monday after the singer’s usual drummer, Steve Ferrone, tested positive for COVID before the show.Mayer’s crew was actually down by two at the start of his second night at MSG, with backup singer Carlos Ricketts Jr. also testing positive.