Well, many would say that a predictor of future performance is past performance, and aside from injury, Ortiz and Bay have been great producers.
Past performance needs to be adjusted for circumstance, and normal performance for a 34 year old DH built like Papi is decline. Papi is past his prime, and any projection should contain that.
Youkilis's past, OTOH, shows that hes still improving, and just entering his prime.
Bay should be great though.