Barrera threw only three pitches in an intrasquad game Friday before walking off the mound with pain in his right elbow. The hard-throwing rookie had elbow surgery in 2009, throwing only 3 2/3 innings that season.
"Once you have Tommy John surgery, you think the worst, but it was good news," Barrera said Saturday.
He said the MRI revealed a big bone bruise on the back of his elbow and a strained flexor tendon.
"Hopefully there are no more scares like that," Barrera said. "They said it's going to be fine, so I'll hope for the best."
Barrera said he would not be allowed to throw for two to three weeks.
"We'll do icing, just for the sake of the strained flexor muscle," Barrera said. "Other than that it should be fine."
Barrera said he felt pain after throwing a fastball to Chris Getz and walked off the field, not wanting to throw any more pitches and possibly doing more damage.
Barrera returned to the mound last June 15 and finished the season going 3-1 with a 1.80 ERA, holding opponents to a .193 batting average and striking out 25 in 25 innings in 16 relief appearances with Double-A Northwest Arkansas.
"I was throwing 95-plus," Barrera said.
Barrera said he felt fine warming up, but when he tried to dial up the velocity in the intrasquad game he felt something in his arm.
"I just went too fast, too hard, too quick," he said. "I should have backed off a little bit. I should have taken my time a little bit. I just have to get to the mentality to just calm down and not go 110 percent all the time."
The Royals open exhibition play Sunday against the defending American League champion Texas Rangers. Manager Ned Yost announced his starting lineup, which could be similar to the March 31 opener against the Los Angeles Angels.
Yost said the starters would go four or five innings, get a couple of at-bats and then he would substitute.