"The Diamondbacks beefed up their pitching staff Saturday by acquiring help at the back of the rotation, and they say they're still hoping to fortify their bullpen in the hours before Sunday's 1 p.m. non-waiver trade deadline.

Right-hander Jason Marquis, acquired from the Washington Nationals in exchange for minor-league infielder Zach Walters, should give the Diamondbacks stability at the back of the rotation, an area where they have cycled through multiple pitchers throughout the season.

Marquis is scheduled to make his Diamondbacks debut Wednesday in San Francisco, though manager Kirk Gibson wouldn't say who will be bumped from the rotation to make room.

"He definitely makes us stronger as a staff," Gibson said of Marquis, who has a 3.95 ERA in 120 2/3 innings. "We have some decisions to make, but we'll figure that out."

General Manager Kevin Towers said all week he had no intention of trading any of his top prospects, and he stayed true to his word with this deal. Walters was posting good numbers in Low-A South Bend, hitting .302 with a .377 on-base and .485 slugging, but several Diamondbacks talent evaluators project him as a utility infielder at the major-league level.

The Diamondbacks will pay all that remains -- about $2.5 million -- of Marquis' $7.5 million salary this season. He will be a free agent at the end of the season and is not expected to be a ranked free agent, meaning he will not return a compensation draft pick.

Marquis missed close to four months last season after having bone chips removed from his elbow. He's been durable other than that, averaging 32 starts with a 4.49 ERA from 2004 to 2009 and making all his starts this season."