SEATTLE -- With right-handed reliever Arquimedes Caminero's arrival on Monday, the Mariners optioned starter Taijuan Walker to Triple-A Tacoma to open up a spot on the 25-man roster.
"It may be different if we were 10 or 15 games below .500," manager Scott Servais said of sending down Walker. "But we're not. Thanks to the guys in that clubhouse continuing to play good baseball, we're a few games over. And it's about winning and who gives us the best chance to go out and compete and win the next time out."
Walker has struggled with tendinitis in his right foot for months. He missed a start in June and then went on the disabled list July 6 before returning Saturday, when he allowed six runs on seven hits in four innings. The pain in Walker's foot was gone, but it wasn't the performance Servais wanted.
Walker has a 4.10 ERA over his past five starts. That's compared to 3.48 ERA in his first 12 and a 1.44 ERA in April.
"Taijuan Walker is a part of our future," Servais said. "A big part of it, just like I said with [starter James] Paxton and [catcher Mike] Zunino. But right now we need to take a step back and do what's best for him, and I'm pretty confident he'll take on the challenge and he'll be back."
Paxton and Zunino both started the regular season in Tacoma. Since being recalled, Paxton has become one of the Mariners' more reliable starting pitchers and Zunino one of their more consistent hitters. 
The Mariners acquired Caminero from Pittsburgh Saturday in exchange for players to be named. The hard-throwing righty has a 3.51 ERA over 39 appearances this season. He has been better in the past month, logging a 2.77 ERA since July 1. He sometimes struggles with control, but when he puts the ball over the plate, he's dangerous.
"It's 162 games. Everyone is going to have ups and downs," Caminero said of his early struggles.
Aríel Miranda will start on Friday in Walker's place. The lefty allowed two runs on eight hits in his Mariners debut against the Red Sox on Thursday.
Worth noting
• The swelling in Paxton's arm has gone down, and his range of motion has improved since talking a comebacker off his left elbow in the ninth inning Sunday. After Sunday's game, the X-rays came back negative. He was expected to see team physician Mitch Storey on Monday evening.
• Ketel Marte was expected to play nine innings with Tacoma on Monday. He has been on the DL with mononucleosis since July 20.