Shiraldi had had a great year, but he had never been very effective all year coming in in relief directly after Clemens. It's because, I think, that Clemen's fastball was just as fast if not faster than Shiraldi, and Shiraldi's fastball was flat, whereas Clemens' fastball moved and darted.
So opposing batters had already been 'geared' to Clemens, and Shiraldi was a step down. Whereas when following the rest of the Red Sox staff (more junkball oriented) Shiraldi looked unhittable.