Seattle Seahawks Star -- WORST DOG OWNER EVER ... Lawsuit Claims

Seattle Seahawks defensive stud Michael Bennett not only abandoned his dog -- he caused the puppy to have an emotional breakdown that destroyed his health, this according to a lawsuit obtained by TMZ Sports.

We broke the story ... Bennett was in deep dog crap with a pet boarding facility in Tampa, FL, where he allegedly LEFT his 4-month-old boxer Koa back in March after he signed with the Seahawks.

The facility claims it made multiple efforts to contact Bennett, but he blew 'em off -- so now, it's taking his ass to court.

In the lawsuit, filed in Florida, the Lucky Dog Daycare & Resort (ironic name for Koa) claims the puppy was so distraught after being abandoned, he "eventually began refusing to eat, losing his hair and clearly failing to thrive."

There's more ... Lucky Dog not only claims Bennett screwed them out of daycare bills for Koa, but says it also had to pay for "costs of arranging a foster home" for the dog.

Lucky Dog is seeking more than $5,000 to make things right. We reached out to Bennett for comment -- but he abandoned our phone call.