Frias is eligible to return to the club Tuesday, after the Monday end of Albuquerque's season. After Sunday's option, Frias threw a bullpen session of starter's length, while Haren did not throw off the mound. Haren would normally throw a session three days before a start.
Haren is coming off a quality start Friday night, when he allowed two unearned runs in six innings. Because Wednesday's game against his former team starts at 12:10 p.m. PT, he would have less than the normal five full days between starts. Mattingly said that was one reason for the decision.
A sixth starter also will provide extra rest for everybody, including Ryu, who just came off the disabled list, and Zack Greinke, who has been dealing with elbow tenderness.
The decision also will line up Greinke, Ryu and Clayton Kershaw to pitch in San Francisco against the Giants on Sept. 12-14.
Haren is 3-1 with a 4.26 ERA against Arizona this year, and 0-1 with a 4.50 ERA against Washington.
Of course, conspiracy theorists should have fun with the notion that pushing Haren back could compromise his chances of throwing 180 innings this year, which would trigger a conditional option for a 2014 salary of $10 million.
Haren has thrown 156 innings, figures to have five starts remaining and would need to average five innings each to guarantee the contract. He is 11-10 with a 4.27 ERA and hasn't missed a start.
Haren reiterated after Sunday's game that he hasn't even decided if he will pitch next year, regardless of contract status.
"I didn't come into this season shooting for a number of innings, I was shooting to pitch good," he said. "If it happens, it happens. I'm undecided about next year regardless of what happens."
Frias last pitched one inning in relief of Haren on Friday night. He has a 5.65 ERA in eight appearances after replacing the injured Chris Perez in the bullpen, but made 20 starts this year splitting time between Albuquerque and Double-A Chattanooga.