Complex sentence: Depending on the source, Manteca, who had been a stalwart of the rhythm section, left either in 1955 or in 1957, retiring as had other members through the years because of health matters.
Split into simpler sentences: Depending on the source, Manteca, who had been a stalwart of the rhythm section, left either in 1955 or in 1957. He retired as had other members through the years because of health matters.