Cook allowed runs in three of his four outings and five runs overall for a 10.38 ERA after being recalled from Nashville on April 26. It's been rough going for the right-hander, a 2012 All-Star who has yet to find his form at the big league level this year, dating to Spring Training.
"He had a couple good innings, he had a couple bad innings," said A's manager Bob Melvin. "It's all about consistency for him and getting him back to where he was for the better part of his duration here as an Oakland A."
Smith posted a 2.31 ERA and two saves in eight relief appearances for the Sounds. He allowed two runs in a loss just Saturday evening, but had surrendered only one in 11 innings prior.
"He's been closing for them and throwing well," said Melvin. "We've been getting good reports on him all year."
Smith said he's been trying to incorporate his changeup and slider more, rather than solely relying on his sinker.
"It's something that's coming along," said Smith, who made his big league debut with the Tigers last year. "I just have to trust it even more.
"I feel like I'm a better pitcher this year than I was last year."
The A's remain without a second left-handed reliever in the wake of Eric O'Flaherty's injury. The lefty is scheduled to have an MRI on his shoulder on Sunday and will be further evaluated Monday.
In the meantime, the A's hope to get southpaw Fernando Abad, owner of a 7.04 ERA, back on track.
"Probably down a little bit on his velocity," said Melvin. "He's going through a rough stretch, and he's not the only guy. We'll continue to work with him, continue to get him confidence."
Jane Lee is a reporter for MLB.com. Read her blog, Major Lee-ague, and follow her on Twitter @JaneMLB. This story was not subject to the approval of Major League Baseball or its clubs.