"He needs consistent work to get consistent with his command and the action on his pitches," manager Eric Wedge said of Kobayashi.
Aquino joined the organization as a Minor League signee and invitee to Spring Training camp. He has 135 Major League relief appearances under his belt with the Diamondbacks, Brewers and Orioles, compiling a 2-4 record and 5.40 ERA with 17 saves. He's struck out 132 and walked 72 in 138 1/3 innings.
The 31-year-old Aquino had some shoulder issues at Columbus this year but allowed just one run on one hit in eight innings over eight outings. He saved four games in as many chances. Triple-A batters hit just .042 (1-for-24) off him, and left-handers were 0-for-12. He was on the DL with right shoulder tendinitis from April 16 to May 6.
Wedge got a look at Aquino in Spring Training and had mixed emotions.
"He has a good arm and good stuff," Wedge said. "He was just a little erratic at times."
The Indians, desperate for relief help, are willing to take a chance on Aquino, just as they are with Matt Herges and the newly signed Luis Vizcaino.
To make room for Aquino on the 40-man roster, the Indians transferred left-hander Scott Lewis (strained forearm) from the 15-day to the 60-day disabled list.
Sipp appeared in nine games for the Tribe since his April 22 callup from Columbus. He provided a boost to the bullpen early in his tenure with the Tribe but has battled control problems in his last three outings. In all, he compiled a 4.26 ERA in 6 1/3 innings of work, striking out nine and walking eight.
"He needs to go down and relax and get himself back on track so he can come back and help us," Wedge said.
Sipp's demotion leaves the Indians without a true matchup lefty in their bullpen. The only left-handed reliever is Aaron Laffey, who is used as a length option out of the 'pen.
With Huff and Aquino on board, the Indians will have used 20 different pitchers this season, including eight starters.