If Diaz is so terrible why would the Jays want him? You don't think that the Cardinals traded him because he was blocked and wasn't going to start? What makes you convinced last year (when he was injured) is closer to his true talent level, and not the year before?
IMO the situation is set up to give Diaz a chance to succeed - he won't initially start but should get lots of playing time over the season.
Gurriel has only just turned 24 and was given a 7 year contract - he's not a normal prospect out of college and he had already been playing professional baseball when we signed him. I've always thought he'd be fast tracked to the majors, though his injury problems last year have undoubtedly set him back.
It's strange that you write off the first 2, but not Gift, who has only played 28 games in the majors.