Kaplan replaces Rome Hartman, said CBS News President Sean McManus. Hartman had held the position since before Couric began at CBS last September.
The newscast has been a distant third in the ratings behind ABC and NBC. During last month's pivotal ratings "sweeps" period, Couric's average of 7.6 million viewers was 6 percent down from what Bob Schieffer recorded in February 2006.
More troubling was the newscast's apparent confusion in direction, driving viewers away from the anchor given a multimillion-dollar commitment to jump from NBC's "Today" show.
McManus said he believes the broadcast is improving and increasing its focus on breaking news, but needed a strong executive like Kaplan to take it to the next level.
"I'm not sure that we are consistent enough so that every single night you know what you are going to get from the `CBS Evening News,"' he said.
McManus said he decided less than a week ago that a change was needed and reached out to Kaplan to hear his ideas. Deciding he liked them, he brought Kaplan's name to Couric to get her assent.
He said he doesn't believe Couric's performance has anything to do with the ratings problems. "She is very good at this and will be even better at it," he said.
"Her ability and qualifications are not an issue with me at all," McManus said. "Creating the right show around her is what is important."
CBS' shift comes less than a week after NBC News announced that the executive producer of "Nightly News," John Reiss, was leaving. Longtime ratings leader NBC and anchor Brian Williams have been sliding in the ratings as ABC's "World News" with Charles Gibson has won for three of the past four weeks.
Before moving into senior management, Kaplan won 34 Emmy Awards as a producer at ABC News. He worked there with Paul Friedman, currently a key deputy to McManus. A large, opinionated man with a booming voice, Kaplan was also a good friend of President Clinton.
He led CNN's domestic operations from 1997 to 2000 as the cable network began to lose audience to the growing Fox News Channel. After teaching at Harvard and briefly returning to ABC, he ran MSNBC from 2004 until leaving last summer, sharpening its programming and setting the stage for modest ratings gains.
Couric took over from Schieffer shortly after Labor Day last year. Her mandate was to shake up a fossilized evening news format that differs only slightly among the three networks. For her first two weeks, Couric led in the ratings. But she hasn't returned there since.
At first, the broadcast emphasized longer, prepared pieces with less focus on breaking news; Hartman came to the evening news from "60 Minutes." Couric's talent as an interviewer was showcased and a regular feature, "Free Speech," invited outside commentary from well-known people like Rush Limbaugh as well as average Americans.
But the commentary bombed and was shelved after two months. Couric's interviews have also been de-emphasized after some were criticized for running too long.
The show has gradually shifted more toward a traditional breaking newscast, but critics say it often misses stories. Many in the industry believed too many people were offering input and there wasn't a clear sense of who was in charge.
"The lesson of the last six months is that it's very difficult to reinvent the wheel," said Andrew Tyndall, a consultant who monitors the content of broadcast evening newscasts.