Well, no matter how you paint it, Cook is not an ace. Heck, his PECOTA comparables are 1998 Scott Erickson, 1972 Mel Stottlemyre and 2008 Jake Westbrook.
That's an innings eater that has quite a few decisions, because he works into games and gives up about four runs an appearance. It's not a bad thing. All teams need that. But when he's starting opening day, it's not a good sign.