Alex Rodriguez has shoulder strain?

CHICAGO -- New York Yankees slugger Alex Rodriguez has been battling a left shoulder injury of unknown severity for the past couple of weeks, an injury that may be the reason for his lower-than-average power production.

"It's just a small issue that only bothers him when he dives for a ball," manager Joe Girardi told ESPNNewYork.com after having a conversation with Rodriguez shortly before Sunday's game between the Yankees and Chicago Cubs at Wrigley Field.

But another clubhouse source, who asked that his identity not be revealed, acknowledged that Rodriguez has been suffering from what the source called "a strain" for several weeks.

"He's managing it as best as he can," the source said.

Girardi, however, said Rodriguez is playing with a sore shoulder, not a strain.

"Guys are sore and a little beat up," he said. "No doubt about it. ... No, it's not a strain."

Following the Yankees' 10-4 win over the Cubs on Sunday, however, Rodriguez said his shoulder feels OK and he is getting treatment, calling it part of the normal wear and tear during a long season. He had three hits and scored three times during New York's victory.

"You go through regular bumps and bruises in a long season," he said. "I think that's just another small example of it. But for the most part, I feel pretty good."

He said he has been getting treatment on his shoulder for about 10 days, but called it "nothing out of the ordinary."

"It feels OK," he said.

Before Sunday's game, he was seen with his left shoulder heavily wrapped in what appeared to be large ice packs. He also spent a long time in the trainer's room and could be seen lifting 10 pound dumbbells under the supervision of Yankees conditioning coaches about two hours before the game.

Asked about Rodriguez's unusual pregame routine, Girardi said he was unaware of any injury. But while the team took batting practice -- Rodriguez took his normal turn in the cage -- the manager returned to say that Rodriguez had been applying heat, not ice, to the area and acknowledged that there was, indeed, an injury of some sort to the third baseman's left shoulder.

Rodriguez has played in 66 of the Yankees' 70 games so far, including a 10-4 win over the Cubs on Sunday night. But while he entered Sunday batting a fairly productive .282, with 42 RBIs, his power numbers are indisputably down, even in comparison with his diminished production of the past two seasons since he underwent surgery to repair a torn hip labrum before the 2009 season.

Three Yankees -- Curtis Granderson, Mark Teixeira, both with 21, and Robinson Cano with 14 -- have hit more home runs than Rodriguez, who has 13, and his slugging percentage, .506, is also lower than all of theirs.

And he is on pace to hit fewer home runs this season than he has since 1997, when he hit 23 as a 21-year-old Seattle Mariner. Although over his career he has hit a home run every 14.5 at-bats, this season his rate had been one in every 19 at-bats entering Sunday. That was lower even than his rate in 2009, when returned from the hip surgery in May and still hit 30 home runs in just 444 at-bats.

Asked if an injury to the left shoulder of a right-handed batter could rob the batter of some power, hitting coach Kevin Long said, "It could, because it could affect his finish and the way he follows through on his swing."

Asked if such an injury might be affecting Rodriguez, Long said he was unaware of any injury and didn't see any signs of it.

But the numbers tell a different story, and now, so do the Yankees. Alex Rodriguez's shoulder is hurting. The question is, how badly?

Wallace Matthews covers the Yankees for ESPNNewYork.com. Information from The Associated Press was used in this report.