Los Angeles Angels star Mike Trout is dealing with a "rare condition" in his back that he likely will have to manage "through the rest of his career," according to head athletic trainer Mike Frostad.

Trout was placed on the injured list on July 18 with rib cage inflammation, which is creating pain in his back, but the Angels initially expected the three-time MVP to return quickly.

Frostad told reporters, however, that the Angels now are concerned because Trout has been diagnosed with a costovertebral dysfunction at his T5 vertebrae.

"This is a pretty rare condition that he has right now in his back," Frostad said Wednesday. "The doctor [Robert Watkins III], who is one of the most well-known spine surgeons in the country -- if not the world, doesn't see a lot of these.

"And for it to happen in a baseball player -- we just have to take into consideration what he puts himself through with hitting, swinging on a daily basis just to get prepared, and then also playing in the outfield, diving for balls, jumping into the wall -- things like that. There's so many things that can aggravate it. But this doctor hasn't seen a lot of it."

Trout, 30, had a cortisone injection in his back last week and had not resumed baseball activities as of Wednesday, according to Frostad, who said the 10-time All-Star is "upbeat" but acknowledged the possibility of a long-term condition.

"I think he's starting to feel like he's getting the benefits," Frostad said. "But long term, we do have to look at this as something that -- he has to manage it, not just through the rest of this season, but also through the rest of his career, probably."

Trout is scheduled for a follow-up appointment with Watkins next week, and Frostad said the Angels have not yet reached the point where they must decide whether to shut him down for the rest of the season.

Trout, the second-highest-paid player in the game at $37.1 million, has not spoken to reporters since last week's All-Star Game. He had been enjoying a nice bounce-back season after a calf injury limited him to just 36 games last season, batting .270 with 24 homers and 51 RBIs in his first 79 games while providing a rare bright spot in what has been a dismal season for the Angels.

"He's been a great teammate," Angels interim manager Phil Nevin said. "He's been [in] the dugout, helping out his teammates -- he's obviously a good sounding board for a lot of young players. For them to have him here and know that he's supporting them is huge, I'm sure, for some younger guys."

The Associated Press contributed to this report.