If you are pitching on normal (4 days) or extra (5+ days) rest, you generally pitch a 30-50 pitch bullpen session halfway between starts. It keeps you sharp yet doesn't wear you out. If you know in advance that you are pitching on short (<4 days) rest, you skip the bullpen session. Apparently, it was right after Santiago had his bullpen session yesterday that the Sox decided to scratch Sale and pitch Santiago on short rest. So the issue isn't so much that Santiago threw 80+ pitches 4 days ago, but that he also threw 40+ pitches yesterday.