Watson is a very good 4th line player. He's not just an interchangeable fringe player who has no role, like many teams employ on their 4th lines. He can regularly chip in 10+ goals, and he has been an effective penalty killer for us in past years. He's a very good, very physical presence on the 4th line. He'll fight, but he's not good at it. His real value is in terms of finishing every check, making opposing defenders "hear footsteps" when he's on the ice.
He was the leading hitter for the Preds for the past 4 years. And the Preds are a very soft team that really needs a physical presence. So this will hurt the Preds badly IMHO. It's an easy win for the Sens and mindless trade for the Preds. Unless they are going to sign Matt Martin or some other heavy hitter to replace him.
Last season the Preds faced chaos on so many fronts it's hard to count. Watson was also not effective. He got caught behind the play, he lost that crucial second or two which separate his hitting from being effective into just making him fall behind. He lost the effectiveness on the forecheck. There was no physical reason for this, but almost every player on the Preds faced some sort of personal effectiveness crisis like this, so I don't think it's entirely fair to single out Watson for his own issues.
He had a bad year. Granted. But he'll bounce back and be a very useful component for the Sens IMHO.