SEATTLE – Mariners rookie center fielder Braden Bishop was placed on the 10-day injured list on Wednesday afternoon after tests revealed a lacerated spleen, which is why he’d been forced to come out of Tuesday night’s 11-5 loss to the Astros in the fourth inning after feeling intense pain in his left shoulder.

The injury initially was believed to be a cramp in his trapezius muscle, but further tests showed the 25-year-old was dealing with the internal injury. Lacerated spleens can cause pain in the shoulder, as well as dizziness, both of which Bishop dealt with Tuesday after coming out of the game after catching a fly ball in the fourth inning.

Bishop took a fastball in the ribs during a game with Triple-A Tacoma last weekend, but it’s not certain yet if that is what caused the injury.

The Mariners don't know yet how long Bishop might be sidelined.

Bishop, the Mariners’ No. 11 prospect per MLB Pipeline, was making his first start after being recalled from Tacoma on Sunday after Jay Bruce’s trade to the Phillies. The former University of Washington standout had gone 2-for-24 at the plate in 10 games this year in his first action in the Majors and posted a .293/.378/.500 line with seven homers and 29 RBIs in 38 games at Tacoma.

To fill Bishop’s spot, the Mariners added outfielder Mac Williamson, who was signed as a Minor League free agent on Wednesday. The 28-year-old had declined a Minor League option by the Giants on Saturday after being designated for assignment.

Williamson has played parts of five seasons in the Majors with the Giants, batting .207 with 14 home runs and 40 RBIs in 135 games. He hit .118 (6-for-51) in 15 games for San Francisco this season and posted a .378/.459/.756 line with nine homers and 22 RBIs in 23 games for Triple-A Sacramento.

Bautista added to bullpen mix

Right-handed reliever Gerson Bautista, one of the three promising prospects acquired from the Mets in the Robinson Cano/Edwin Diaz trade last winter, was activated off the 60-day injured list by the Mariners on Wednesday to bolster a short-handed bullpen.

Andrew Moore, who made a spot start on Tuesday in place of Yusei Kikuchi, was optioned back to Double-A Arkansas to open a spot for Bautista.

Bautista has been on a Minor League rehab stint with Triple-A Tacoma after recovering from a strained right pectoral muscle sustained during Spring Training.

The 24-year-old features a high-90s fastball and showed a developing slider in the spring and should help a relief crew that has seen 25 different players take the mound already this season.

Two more relievers -- Connor Sadzeck and Chasen Bradford -- just landed on the 10-day injured list this week with strained flexor mass muscles in their right elbows.

Bautista posted a 2.57 ERA with one save in seven innings over five outings at Tacoma after allowing just one hit in five innings in five games at Class A Advanced Modesto to start his rehab.

Bautista is the Mariners’ No. 26-ranked prospect, per MLB Pipeline. He was acquired from the Mets along with outfielder Jarred Kelenic (No. 2 prospect) and right-hander Justin Dunn (No. 3), as well as veterans Jay Bruce and Anthony Swarzak. Bruce and Swarzak were included largely to help offset some of the $120 million of Cano’s salary, and both have already been traded away.

Worth noting

• Kikuchi will slot back into the rotation on Saturday in Anaheim. The Mariners will stick with Marco Gonzales in Friday’s opener of that series, but push Wade LeBlanc back a day to Sunday in order to fit Kikuchi back in after his brief layoff.

LeBlanc pitched eight innings in relief after the Mariners tried an “opener” for the first time in his last outing on Monday. Servais said it’s possible he’ll use an opener again for “a few” of his starters, but declined to get into specifics.

• Felix Hernandez threw well in his first opportunity off the bullpen mound on Tuesday and he’ll throw off the mound again Thursday in a two-inning type situation, then advance to a live batting practice before heading out on a Minor League rehab stint as he works back from a right lat strain.

• Infielders Ryon Healy, Dee Gordon and J.P. Crawford are all progressing well from their injuries and are expected to advance to Minor League rehab outings next week in Tacoma or Class A Advanced Modesto and potentially rejoin the Mariners in Oakland at the end of their upcoming 10-game road trip.