(John McDonnell / The Washington Post)

After Dan Haren’s dreadful season reached a nadir Saturday afternoon, the Nationals will place him on the 15-day disabled list, General Manager Mike Rizzo said. After Haren’s ERA rose to a league-worst 6.15 ERA yesterday, the Nationals became convinced the hip and back ailments that raised red flags over the winter had derailed his mechanics.

“Watching him, the arm gets flat coming through,” pitching coach Steve McCatty said. “When you open up – which he has a tendency because of his back and his hip – if you get flat, [the hitter] sees the ball better. Your stuff’s not quite as sharp. That’s something we’re trying to work on.”

Haren will be placed on the disabled list for only the second time in his distinguished, 11-year career. Last year, he spent 15 days on the disabled list with the Angels with lower-back tightness, which was likely related to a hip ailment he had pitched through before in his career.

“The back issue was something that was a concern to us,” Rizzo said this winter, after the Nationals signed Haren to a $13 million, one-year deal. “Once we took a look at the MRIs and the films and our doctor put his hands on him and saw the condition of his back and hip, he signed off on him.”

Both Haren and the Nationals expressed full confidence in his health through the spring and early in the year. After his start Saturday, Haren said he felt typical “aches and pains,” but nothing that he could blame his poor performance on.

“In the past, I know he had a little problem with his hip and his back,” McCatty said. “When you have a problem somewhere else, sometimes it transfers to another part of your body. If you’re hurting somewhere else, sometimes it can take an effect on your elbow, shoulder. We just got to make sure he feels good.”

Haren, by most statistical measures, has been perhaps the worst start in baseball this year. Opposing hitters have clobbered 19 home runs and compiled an .879 on-base-plus-slugging percentage against him, both the highest totals in the National League. He is 4-9, and the Nationals have lost his past eight starts.

“It’s never easy to sit there and watch a guy you know is a warrior, a competitor, that goes out there every chance he gets,” McCatty said. “He never says a word. He never complains. It’s really tough. All you can continue to do is work on it, be in his corner.”

The Nationals have yet to announce a rotation replacement. Ross Ohlendorf, who relieved Haren yesterday and has allowed two runs over 11 innings this season, figures to be a leading candidate. Because of an off day Monday, the Nationals could wait until June 30 before inserting a replacement.

Depending how long Haren remains sidelined, or how confident the Nationals are that he can recapture his form, the Nationals may try to bolster their rotation with a trade.