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Los Angeles pitcher Dan Haren is scheduled to start Saturday against the New York Yankees. ((Hannah Foslien/Getty Images))

Los Angeles Angels right-hander Dan Haren cut short a throwing session in the bullpen Wednesday after feeling discomfort in his lower back.

"The lower back kind of tightened up," Haren said after the Angels' 2-0 loss to the Kansas City Royals. "It was in the middle of my pitching motion. I went down, I think to my knees. It came out of nowhere. It hurt enough for me to go down. "

Haren is scheduled to start Saturday against the New York Yankees, but manager Mike Scioscia said he's not sure whether that will happen.

"The bullpen was going really well. I was loose," Haren said. "It feels better now than when it happened. We got on it and treated it right way. There's nothing structural."

The team plans to have Haren examined before making a decision on his next start.

"It depends on tomorrow," Haren said. "I'll wake up tomorrow and go in and get evaluated. I did a lot of work today. Thursday was going to be an off day anyway. Time will tell."

The 30-year-old Haren has been one of the American League's top pitchers this season, going 5-2 with a 2.29 earned-run average in 13 games — 12 of them starts. He's among the league leaders in innings with 86 1-3, strikeouts with 74 and opponent batting average of .212.

We're going to take this thing one step at a time and get him evaluated back in California," Scioscia said. "Dan felt a twinge in his back throwing a side piece. We didn't get specific about what he was feeling. He was hurting. We don't know to what extent.

"He says he'll be fine, but we'll get him evaluated. He walked off, so that's always a good sign. Things like that are part of a major league season. That's why we have a depth chart to hopefully absorb it."