OAKLAND -- Less than 24 hours ago, New York Yankees rookie outfielder Slade Heathcott said he thought his right quadriceps injury was so minor that he would be back out on the field within a day or so.

Now, however, it turns out he probably won't play for a month at the very least.

The Yankees placed Heathcott on the 15-day disabled list after an MRI revealed he has a Grade 2 strain of his quad, an injury that generally takes a minimum of a month to heal. The Yankees had called Heathcott up from Triple-A Scranton on May 20 to replace the injured Jacoby Ellsbury.

In his place, the Yankees will recall OF Ramon Flores, who is batting .294 with four home runs and 15 RBI for Scranton.

"I was kind of surprised when I heard the diagnosis,'' said Heathcott, who batted .353 with a home run and three RBI in six games with the Yankees. "I thought it was something that was manageable, just a little bit more sore. Obviously there's a reason why.''

Heathcott said he believed he aggravated the injury, which has troubled him on and off since spring training, chasing a Prince Fielder home run against the Yankees in a game on May 22. Heathcott has an extensive injury history -- two shoulder surgeries and two knee surgeries -- and believes the injury may have resulted from the latest knee surgery, performed in the off-season.

"I think my body is still adjusting to the last surgery I had and it might just take a little more time,'' he said. "Obviously it's frustrating, but it's just another speed bump to me. I've been up and down, faced a lot bigger challenges than a quad.''

In Heathcott's absence, Joe Girardi said Brett Gardner, who had been shuffled to left field with the acquisition of Ellsbury before the 2014 season, would return to centerfield when Flores, a corner outfielder, is in the lineup.