The Seattle Mariners opened up their trade deadline season on Saturday night by sending three-time All-Star Luis Castillo to the Chicago White Sox for three players, including reliever Seranthony Dominguez, and some salary relief.

While prospect Boston Smith is likely the best overall player that Seattle received, the immediate benefit comes from adding Dominguez to an increasingly thin Mariners bullpen.

Seattle has operated with a seven-man bullpen, not eight, for the last two months or so, as a result of its "piggy back" and six-man rotation strategies.

In the seven-man bullpen, only Andres Munoz, Gabe Speier, Jose Ferrer and Eduard Bazardo have been reliable leverage options, but they've shown cracks of late.

Ferrer has allowed runs in his last two appearances, including allowing home runs to Royce Lewis on both Friday and Sunday of this most recent series against the Minnesota Twins. Bazardo has allowed runs in four of his last six appearances, and that doesn't factor in inherited runs he allowed in another of those six. He's allowed at least one hit in eight of his last nine appearances, meaning traffic has been constant against him.

Dominguez is not a perfect answer for this bullpen, but he is another leverage-able arm that is capable of helping bump Bazardo back down the pecking order a bit.

So, here's what you need to know:

Now 31 years old, Dominguez is in his eighth year with the Philadelphia Phillies, Baltimore Orioles, Toronto Blue Jays and White Sox. He's been traded in each of the last two deadlines, so he knows the drill.

Lifetime, he's 27-26 with a 3.55 ERA. This season for Chicago, he's gone 4-3 with a 3.99 ERA in 43 games, registering 12 saves.

On the positive side, his fastball velocity gets up into the upper-90s and he misses bats, something the Mariners need more of in the bullpen. He's fanned 47 batters in 38.1 innings this year.

On the negative side, he's walked 23 batters in 38.1 innings, meaning he walks more than one every two frames, which has led to a WHIP of 1.33. He's certainly not someone that can be fully counted on for clean frames.

It's just the reality.

With Castillo gone, the Mariners have the ability to go back to a five-man rotation and a full eight-man bullpen.

That bullpen is seemingly:

Munoz, Speier, Bazardo, Dominguez, Ferrer, Alex Hoppe, Michael Rucker and Cole Wilcox.

The Mariners do need to add additional bullpen help somewhere:

The options? Make another trade, or simply wait for Carlos Vargas and Matt Brash to get healthy some time in August, and promote Kade Anderson from the minor leagues.

That's one way to get to eight, though it's certainly not the cleanest, since you don't know if Vargas and Brash will come back fully healthy.

The trade deadline is Monday at 3 p.m. PT.