Nick Piecoro

USA TODAY Sports

PHOENIX — The Diamondbacks made a surprise addition to their bullpen on Friday, agreeing to terms with left-handed reliever Oliver Perez on a two-year deal, according a baseball source.

Contract language still needed to be hammered out and Perez must pass a physical exam, but his addition would give the Diamondbacks another power arm in their bullpen, one that historically has been a tough assignment for left-handed hitters.

Club officials declined comment until the deal becomes official.

Once a promising starting pitcher, Perez reinvented himself with the Seattle Mariners as a reliever the past two seasons. After posting a 2.12 ERA in 292/3 innings in 2012, Perez had a 3.74 ERA with 74 strikeouts in just 53 innings last season.

The addition muddles the Diamondbacks' projected bullpen construction. It would appear to be bad news for left-hander Joe Thatcher, who struggled last season after being acquired from the San Diego Padres at the trade deadline.

This week, manager Kirk Gibson sounded frustrated by the performance of his left-handers, going so far as to say the club might not open the season with a lefty in the bullpen.

In 22 appearances with the Diamondbacks last season, Thatcher had a 6.75 ERA and opponents posted a .429 on-base percentage against him. He has allowed one run on two hits and a walk in 22/3 innings in the Cactus League. He gave up an RBI double to the lefty-swinging Josh Reddick on Thursday at Salt River Fields.

Since joining the Mariners, Perez has rediscovered the lively fastball that once made him a top prospect. His fastball has averaged 93.7 mph and 92.2 mph in 2012 and 2013, respectively.

Perez, 32, has been tough on lefties throughout his 11-year career. Last season, he held left-handed hitters to a .238 average and .645 OPS, roughly in line with his career numbers (.230, .687).

This will mark the second time Diamondbacks General Manager Kevin Towers has signed Perez, who signed with Towers' Padres as a 17-year-old in 1999.

Nick Piecoro writes for azcentral sports.