Baseball coaches are pretty worthless when in comes to the MLB. The only thing they do is make the lineup (which is almost the exact same every night) and figure out when to put in a certain pitcher.
Mariners could have gotten Dusty Baker, Joe Torre, or La Russa and it wouldn't matter because the talent blows. You think this guy that they hired really had anything to do with Miguel Cabrera hitting the shit out of the ball? When you get to the MLB you don't need to be coached on your swing or your fundamentals on defense/pitching. Look at the world series this year, it was a 3rd year head coach vs a 1st year head coach.
Mariners being worthless is a talent issue, which stems from shitty trade after shitty trade (Adam Jones for Erik Bedard!!!11!) and having horrible scouts when it comes to the draft. Look at some of the players they passed over for fucks like Morrow.