Photos on the Web site "The Cheater's Guide to Baseball Blog" appeared to show how Rodriguez, in two games against the Texas Rangers, habitually adjusted the brim of his hat before pitches -- and that there was a white substance visible under the brim.
Major League Baseball would not confirm if any complaints were filed by the Texas Rangers, who faced him twice during the Angels' season-opening three-game sweep. But the Dallas Morning News reported Thursday it had confirmed MLB was investigating.
But Rodriguez had an explanation, and showed reporters his hat before Thursday's game to prove he was innocent.
"It's from the rosin bag," Rodriguez said before Thursday game, according to the Los Angeles Times. "If that's illegal, why is there a rosin bag on the mound? You can come check me any time you want. I have nothing to hide."
"If somebody sees something on my hat, they're probably going to think I'm cheating. But I'm not, so I'm not worried about that," Rodriguez said. "They're looking at it in a bad way and trying to make something out of nothing.
"It's stupid -- it's rosin," Angels manager Mike Scioscia said, according to the Times. "You touch the rosin bag and adjust the brim of your cap, chances are you're going to get a white buildup there. That's what it is."
Rodriguez told the Times he is fidgeting with his hat more this season because he switched from wearing sports goggles to contact lenses.
"Before, I used to adjust my glasses a lot," Rodriguez said, according to the Times. "I just do that normally. I don't see it as any big deal. I don't get it. What do I have to do, not touch any part of my body? It's not like rosin is going to make me throw harder."
If evidence shows Rodriguez violated rule 8.02b by applying a foreign substance to the baseball, he could face a suspension of up to 10 games. And after last year's World Series, baseball is paying more attention to that rule.
In Game 2 of last year's World Series, cameras showed a brown substance on Kenny Rogers' left (throwing) hand in the first inning, and St. Louis manager Tony La Russa brought it to the umpires' attention. Rogers' hand was clean when he came out for the second inning.
Rogers insisted then that mud, resin, spit and dirt was what everyone saw at the base of his left thumb -- not pine tar or anything else illegal.
In Febuary, baseball's playing rules committee approved changes that would eject and suspend players for intentionally defacing or discoloring a ball. Umpires have the discretion to issue only a warning if they determine a pitcher didn't intend to alter the characteristics of a pitch. Previously, such a pitch was called a ball, a warning was issued and the violation was announced.
Information from The Associated Press was used in this report.