It'snot necessarily a fact the the Bruins management was unhappy with Donato, I'm sure they understand it can take time to learn to play the game at the NHL level. Donate was given many opportunities to show that he was ready, but unfortunately his play was extremely inconsistent. At this point in time the Bruins need consistancy especially if they hope to advance in the playoffs. Donate may have potential (or not) to be the better player in this deal, but we may not know for another 3 years and the Bruins need help now.
For me this deal is very ordinary, Coyle is not a core player he is more of a support player, while he has some size and will throw the occasional check he is not a power forward nor a consistent scorer. This is a conservative safe move to add depth, experience and consistency to the lineup. Basically, the Bruins know what they will be getting from Coyle on a nightly basis . . . Which is what they need at this time.