The right-hander blew his fourth save in 10 chances on Wednesday against the Chicago White Sox, and was 1-3 with a 7.56 ERA in 18 games this season.
Borowski had 45 saves last season but has struggled since returning from an arm injury. He returned to the Indians in late May after being sidelined for about a month with a strained right triceps that caused him to lose velocity on his pitches.
General manager Mark Shapiro said the club tried to give Borowski every chance but the reliever wasn't able to return to form.
Shapiro said he's not confident that anyone in the bullpen can emerge as the team's future closer, leaving it an issue that likely will be addressed in the offseason.
Manager Eric Wedge said he expects to give the majority of save opportunities the rest of this season to Masa Kobayashi, who along with Rafael Betancourt, has four saves.
"I want to pick spots where we try other people, too," Wedge said. "It's not going to always be Masa, but the bulk of it will go his way."
Shapiro also said the 2008 season has been a disappointment -- the Indians are in last place in the AL Central -- but he doesn't see the need for a total rebuild.
He continues to talk with other teams about possible trades but offered no specifics.
"We've been active for a long period of time, and the calls are coming both ways," he said.
Shapiro also declined to say if the team is making another push to re-sign AL Cy Young Award winner C.C Sabathia, who rejected a $72 million, four-year extension from the Indians during spring training and announced he wouldn't negotiate any deal until after the season.
Wedge echoed Shapiro's thoughts on the rebuilding issue.
"I still feel like we got our core in tact," said Wedge, whose team currently is without Travis Hafner, Victor Martinez, Fausto Carmona and Jake Westbrook because of injury. "I expect our expectations to be right back where they should be at the start of the season next year. We've got a lot of good players. A lot of them are injured now, but we have a lot of good players."
Lewis has split the season with Buffalo and Cleveland. He was 1-2 with one save and a 3.60 ERA in 11 relief appearances with Buffalo. Slocum, who has been converted to a reliever, had one save and a 0.69 ERA in seven relief outings with the Bisons.
The Indians also designated right-hander Rick Bauer for assignment. He had a 13.50 ERA in four relief appearances.