Zach Eflin was just 22 years old when he reached the major leagues, and it would have been hard for his debut in June 2016 — nine runs in less than three innings — to go any worse.

Eflin's season didn't exactly end the way every rookie dreams: He had surgery on both knees.

The righthander had pitched most of his life with chronic knee pain, which made his introduction to the majors even more stinging.

Eflin spent the next year shuffling between the Phillies and the minors, and between the active roster and the disabled list. Nothing seemed easy once he reached the major leagues.

But it was those struggles, and the lessons provided, that made his tremendous performance last month possible. Eflin won all five of his starts and totaled a 1.76 ERA in June, with 28 strikeouts in 30 2/3 innings. He started the season in triple A but has emerged as a key piece of a starting rotation that is helping the Phillies defy expectations.

"You can take it however you want," Eflin said of his June production. "But I always felt that I belonged here regardless of how I had thrown. That's the feeling I always had. But to go out there and put up a very good month is definitely rewarding."

Eflin had a 5.85 ERA through his first 22 major-league starts, but sprinkled in those struggles were glimpses of what he showed in June. He pitched two complete games in July 2016, a month before his surgery. He logged seven innings in three straight starts last season, and three weeks later, he gave up 22 runs over 15 innings.

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Eflin, even when times were rough, offered some promise. His knees proved to be too much of a challenge. In 2016, it was the pain. In 2017, it was the recovery from the surgery, which hindered the way he prepared for the season. And now, 23 months after surgery, he is finally healthy. He has since gained 25 pounds and hopes to add 15 to 20 more.

"When I first got called up, I was in a lot of pain," said Eflin, who will start Tuesday night when the Phillies open a two-game home series against the Orioles. "I was able to pitch, but it was painful. Being able to be healthy this year has been the biggest thing. Being able to focus on pitching alone is awesome."

The healthy Eflin altered where he stands on the pitching rubber, starting his delivery with his feet on the third-base side of the rubber. The change made the 6-foot-6 Eflin feel more imposing against righthanded hitters and added spin to his slider.

The strength in his knees allowed Eflin to pitch aggressively. He was a sinkerballer who pitched to contact before his surgery. Now, he's evolving into a power pitcher armed with a four-seam fastball that averaged 95.29 mph in June. The 102 swings-and-misses he has recorded over 10 starts this year are just 37 shy of the number he had in 22 starts over the previous two seasons. His strikeout rate has nearly doubled while his walk rate has stayed consistently low. He is no longer relying just on ground balls.

"He's showing signs of evolving into the next step of his career," Jake Arrieta said. "And his stuff and results have dictated that."

If anyone in the Phillies starting rotation knows what evolution looks like, it is Arrieta. Before he won a World Series and a Cy Young Award, he struggled just like Eflin. He was demoted twice to the minor leagues when he was with Baltimore. He faced adversity and overcame it.

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This does not mean Eflin is the next Arrieta. But it is proof that the struggles Eflin faced were not unique. It is the way he overcame them that sets him apart.

"Everyone is going to struggle early, other than [Dodgers lefthander Clayton] Kershaw. That's how this game is," Arrieta said. "It's not easy. It's especially not easy to figure it out the first go-around. Sometimes, it takes a stint in the minor leagues. Sometimes, two or three. Who knows? Everyone's path is different.

"But it makes you appreciate the amount of learning and self-discipline it takes to get to that point in your career. Once you're able to get there, you don't take it for granted because you know the flip side of that. The consistent struggle is not something easy for guys to deal with. It pushes guys out of the game. To be able to persevere through that and be able to grow as a person and a player and a student of the game is rewarding if you're able to get onto that other side."