Roenicke, dismissed as Milwaukee manager in May, on Monday replaced Lorenzo Bundy, who will focus on outfield defense.
Mattingly would not say who decided to make the change or provide the reason for it coming on a first-place team this late in the season.
"We felt like this was the right time," he said. "I'm not going to get into the details of why. It makes a good staff better. It's a comfortable fit for me."
General manager Farhan Zaidi said baserunning has been "a major issue all season," partly because of a lack of team speed, but he added that "the responsibility for our running trouble is on all of us, really. Bringing in Ron with his resume and pedigree, it was more about that."
Mattingly said club president Andrew Friedman gave him a list of available coaching candidates and Mattingly said he felt Roenicke was the best choice.
Roenicke hasn't coached third base since 2005 with the Angels for Mike Scioscia on a staff that included Joe Maddon, who was later hired by Friedman as manager in Tampa Bay. The Angels developed a reputation for aggressive baserunning while Roenicke coached third.
Mattingly said his familiarity with Roenicke has expanded in recent years because of regular Spring Training dinners that also included Scioscia, Joe Torre and Dodgers legend Sandy Koufax, among others.
Mattingly deflected a question implying that Roenicke could be viewed as his potential replacement as manager.
"All I'm worried about is winning," Mattingly said. "This is a pretty good job to have, I'm sure a lot of guys would like it. That's always the case. Those are things to me [that are] so far down the road. Just worry about winning games."
Roenicke said he "never thought about" the perception of his hiring leading to Mattingly's firing.
"Donnie wouldn't have asked me or wanted me to be here if that was a concern," he said. "My coming here was strictly to help out."
Roenicke said he had been contacted by other clubs for off-the-field jobs since his dismissal, but his first contact with the Dodgers was Saturday when Mattingly called. Zaidi said there is no agreement beyond this season.
"I can say with all sincerity, this move is about this year in this role, helping us go as far as we can in 2015," Zaidi said.
Roenicke said he missed being on the field and wants to manage again.
"It's hard to be home when you're not supposed to be home," he said. "If I don't manage, I'd love to coach. Whichever way it goes. I enjoy the most the challenge of it, of getting to guys maybe other guys don't get to."