"It's upsetting," Tropeano said. "It's a tough blow to me. Obviously, I want to be out there. I want to pitch. I want to help this team, and it sucks. It's part of the game, I guess, nowadays, and it's one of those things where you've got to stay positive until you get that diagnosis."
Garrett Richards and Andrew Heaney -- the Nos. 1 and 2 starters, respectively, when the season began -- also sustained UCL tears and initially opted for stem-cell therapy in hopes of avoiding the invasive Tommy John procedure.
Heaney, 25, ultimately opted for Tommy John surgery, and Richards, 28, will have a checkup the first week of August to determine whether he will also need to go under the knife. But Tropeano, 25, must make a quick decision so that he does not run the risk of his rehab bleeding into 2018 if stem-cell therapy does not work.
He can wait until October to have surgery and still have 18 months to be ready by the start of the 2018 season.
Pitchers often come back within 14-16 months.
"I don't have the luxury of having 12 weeks to come up with a decision," Tropeano said. "Right now, we're just playing it by ear. We're going to get that second opinion and find out the true diagnosis."
Tropeano was acquired from the Astros alongside catcher Carlos Perez in November 2014, in a deal that sent homegrown catcher Hank Conger to Houston. Tropeano has since appeared in 21 games (20 starts) for the Angels, posting a 3.65 ERA and a 1.43 WHIP in 106 frames, striking out 9.0 batters per nine innings and walking 3.5 per nine.
The 6-foot-4 right-hander spent most of the 2015 season with Triple-A Salt Lake, but had been the Angels' most consistent starter this season, giving up three earned runs or fewer in 10 of his first 12 starts.
But in start No. 13, against the Rangers on Monday, Tropeano gave up three home runs in the first two innings and exited with soreness in his right elbow. He initially felt discomfort while warming up in the bullpen, but continued to pitch until his elbow stiffened up in the second inning, prompting him to alert the Angels.
"It's tough, man," Angels manager Mike Scioscia said. "Any time you see a young kid that works hard and puts himself in a position and gets sidelined by injury, it's a tough part in this game. It's not the fun part."
Tropeano's diagnosis leaves Hector Santiago and Matt Shoemaker as the only current starters who are slated to make up next year's rotation, making it difficult for first-year general manager Billy Eppler to use one of his starting pitchers to acquire prospects for his thin farm system at the non-waiver Trade Deadline.
In the short term, the Angels will benefit from a Thursday off-day and a couple of internal options.
Tyler Skaggs, 23 months into his recovery from Tommy John surgery but coming off a dominant seven-inning performance for Triple-A Salt Lake, will make what could be his last Minor League start Wednesday and could fill Tropeano's rotation spot. So can Jhoulys Chacin, the veteran starter who was banished to the bullpen two weeks ago, but pitched four scoreless relief innings in Monday's 9-5 win.
Said Scioscia: "Our depth is stretched."
Heaney made one start before tearing his UCL. Richards made six, then was diagnosed with a high-grade tear. C.J. Wilson, making $20 million in his final year before free agency, will finish the season without throwing a single pitch. He underwent shoulder surgery last week.
The rash of injured pitchers -- including Skaggs, who underwent Tommy John surgery in August 2014 and was set back about two months earlier this season -- has prompted the Angels to look internally.
"You analyze everything from the throwing program to how you monitor guys," Scioscia said. "And we're going to do that. I just know we've been very healthy over the course of time here; very few Tommy John injuries. This might be a fluke thing, but naturally you have to look from within to make sure that the process is correct."
Prior to the trio of Tommy John candidates this year, and Skaggs' procedure two seasons ago, only a couple of relievers -- Matt Wise in 2003 and Michael Kohn in 2012 -- have required the elbow-ligament replacement surgery under the Angels' watch. Others, like Sean Burnett and Ryan Madson, were already injured when they arrived. And others, like Tyler Chatwood and John Lackey, required Tommy John surgery within three years of leaving the Angels.
Tropeano, under club control for five seasons after this one, is still hopeful he can avoid it.
"We're going to keep all options open," he said. "Obviously, no one wants to get surgery. If [stem-cell therapy] is a possibility, then we're going to look into it. As of right now, this is the first diagnosis. We're going to get that second opinion, and maybe even a third, and then we're going to figure out a plan."