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4/20/2013 8:34 P.M. ET

Zim to DL as Nats call up top prospect Rendon

NEW YORK -- Nationals third baseman Ryan Zimmerman was placed on the 15-day disabled list Saturday after an MRI revealed a strained left hamstring.

"The doctors said he needs to rest for 10 days and he only had a couple of days [off]," manager Davey Johnson said. "We are not going to take a chance to bring him back early and have him re-injure it."

The Nationals called up third baseman and top prospect Anthony Rendon from Double-A Harrisburg. Johnson said Rendon will start at third base against the Mets on Sunday at Citi Field. Rendon will go back to Harrisburg once Zimmerman is activated from the disabled list.

Rendon played 14 games for the Senators this year and hit .292 with two home runs and seven RBIs. He also had a good Spring Training for the Nationals, going 12-for-32 (.375) with four home runs and 11 RBIs. The Nationals wanted a player who is a natural third baseman, with Chad Tracy and Steve Lombardozzi strengthening the bench. Tracy and Lombardozzi were platooning at third base after Zimmerman hurt his hamstring.

"He's swinging pretty good in Harrisburg, he had a good spring for us," Johnson said. "It's no slight against some of the guys I have here who filled in [during the last couple of days]. But it's good to get [Rendon] some experience here."

Zimmerman started having hamstring problems during the weekend series against the Braves, but he didn't complain until Wednesday against the Marlins, when he was taken out of the game in the seventh inning.

Johnson said Zimmerman has had problems hitting and running the bases lately.

"It's kind of been noticeable with his hitting. He doesn't feel real comfortable landing on his left leg," Johnson said. "I know he is concerned about it. Of course, he is a gamer. He wants to play. I thought he was running a little bit better in Miami, but then he came out of the game late because it was tightening up."

Zimmerman didn't seem surprised by being put on the disabled list. It marks the third consecutive year [fourth overall] that he has been placed on the DL. He will spend the time working on his throwing.

"We knew something was wrong," Zimmerman said. "I can't put the team, put Davey at a disadvantage for seven to 10 days. You can't do that. We'll take the time, get it right. It's only April. We'll be good to go."

Zimmerman is looking forward to seeing Rendon make his big league debut on Sunday.

"He had a great Spring Training," Zimmerman said. "He is one of the most talented guys in our system. Lucky for us, we have guys like that who can step up and do things like this. Anthony is young. Sooner rather than later, he will make an impact on this level on a daily basis. I'll be his biggest fan. I want him to come up and do as well as he can to help us win. That's the ultimate goal as a team."

McCatty wants Haren to be himself on the hill

NEW YORK -- After three games, Nationals right-hander Dan Haren has allowed 12 runs in 13 1/3 innings, which is something the team was not expecting after signing him to a one-year, $13 million deal.

Haren said there is nothing wrong with him physically and that he just has to make better pitches during the course of the game.

"I feel good, so it's just a matter of translating good numbers out there," Haren said. "Baseball is a crazy game. As bad as it is going now, it can change in an instant. I just have to remain positive."

Pitching coach Steve McCatty said that it's important that Haren be himself on the mound. McCatty feels that Haren is trying too hard to fit in with the pitching staff. Haren's fastball has been clocked between 89 and 91 mph, but he is trying to throw harder like the rest of his staff mates.

Haren is known to throw the cutter, but he has been throwing mostly fastballs this season. He hasn't used his curveball that much either, according to McCatty.

"The first thing I have to do is let Dan Haren become Dan Haren," McCatty said. "We talked about it before. In Spring Training, I told him, 'You don't have to do anything different -- do what you do.' Everyone talks about the velocity. That doesn't matter to me. He feels better.

"When you go and try to fit in, you know the other four guys are really good. They throw harder, but he is pressing to fit in. He would talk afterward about the way he is pitching -- using more fastballs than cutters. He hasn't used his curveball as much as he has in the past. It's almost like he is trying to show somebody something. Be who you are."

Worth noting

The Nationals are planning to place Rafael Soriano on the paternity list on Monday. He could miss up to three games. Johnson is not sure who the Nationals will call up during that period.

Bill Ladson is a reporter for MLB.com and writes an MLBlog, All Nats All the time. He also could be found on Twitter @WashingNats. This story was not subject to the approval of Major League Baseball or its clubs.