Greinke suffered a cracked rib when he fell to the floor on a rebound in early 2011, which led the Brewers to audition replacement starters. That resulted in a last-minute call for Estrada, who was marooned in Minor League camp.
Three years later, the 30-year-old right-hander is a big part of the Brewers' starting rotation, set to make his season debut against the World Series champion Red Sox in Boston's home opener.
"I'm thankful every day for the opportunities the Brewers have given me," Estrada said. "Not only that they picked me up in the first place, but that they actually gave me the chance to pitch. With this organization, if you're performing well, they're going to give you a chance."
When he got his chance in March 2011, Estrada didn't know it.
The Brewers had claimed Estrada off waivers from the Nationals a year earlier, and after a strong start to the 2010 season with Triple-A Nashville, Estrada found himself pitching in Milwaukee. But after logging a 9.53 ERA in six relief appearances and one start, his stint ended awkwardly. The Brewers optioned Estrada back to Triple-A on June 1, then had to void the move because Estrada complained of shoulder soreness. They placed him on the 15-day disabled list instead, and Estrada missed the rest of the season.
In October, the Brewers removed Estrada from the 40-man roster and assigned him outright to Nashville.
"When I went to Spring Training [in 2011], I understood what my role was going to be," Estrada said. "I knew I didn't have a chance to make the team. I was prepared to start the season in the Minor Leagues."
Greinke's misfortune changed that. On March 8, 2011, Greinke revealed he had been injured playing basketball, and by March 22, the Brewers were still seeking his replacement. As a last effort before going outside the organization, they called down to the Minor League complex for Estrada, and he delivered 4 1/3 scoreless innings against the Padres.
Still, the assignment felt temporary, Estrada said. He was not even given a locker in the Major League clubhouse, so he retreated to Minor League camp after the game to receive his usual treatment.
"I didn't know about Zack," Estrada said. "All I knew is I was coming over to pitch. I didn't even know I was going to start until I got there.
"A couple of days later, someone said, 'You know Zack is down, right? You're going over there for a reason.' I was like, 'Oh, man.' I remember thinking that it stunk for Zack, but this was my opportunity and I needed to take advantage of it. Obviously, I did take advantage of it."
Five days later, Estrada made the same march across the parking lot at Maryvale Baseball Park in Phoenix to pitch six quality innings against the White Sox, sealing his spot on the Opening Day roster. He made one start in Greinke's stead and stuck for the entire season, making 36 relief appearances and seven starts. The Brewers set a franchise record with 96 wins and made it to the National League Championship Series.
It was the opportunity Estrada needed.
"I feel like I've said that every year: 'Just give me an opportunity,'" he said.
Armed with an average fastball and an excellent changeup, Estrada has primarily pitched as a starter for the Brewers the past two seasons. When he has been healthy, he has been very effective. In 2012, Estrada posted a career-best 3.64 ERA in 29 games (23 starts) and struck out 4.93 batters for every walk, the fourth-best ratio among Major Leaguers who logged at least 100 innings. But he also missed a month with a quadriceps strain.
In 2013, Estrada was a full-time starter for the first time and went 7-4 with a 3.87 ERA in 21 starts. He was 3-0 with a 2.15 ERA in his final nine starts of the season and won Brewers Pitcher of the Month honors in both August and September. But before that, he missed two months with a hamstring injury.
"There's obviously been a lot of injuries," Estrada said. "More than I want."
Estrada took steps over the winter to ensure a healthier 2014. At the end of last year, he viewed his usual offseason protocol with Brewers strength and conditioning coordinator Josh Seligman, who thought Estrada was doing too much heavy weight work to strengthen his legs. This winter, Estrada worked out with less weight and more repetitions, and he increased his focus on stretching before and after workouts.
He feels the difference today.
"If I can give them 180-plus innings this season, obviously I'm doing something right," Estrada said.
Here's another thing Estrada does surprisingly right, given his 89-90 mph fastball: Strike batters out. For pitchers who have worked at least 200 innings over the past two seasons, Estrada ranks fourth with 4.50 strikeouts per walk (behind Cliff Lee, Adam Wainwright and Matt Harvey) and a respectable 17th with 8.82 strikeouts per nine innings, not far behind the likes of Justin Verlander (8.99) and Clayton Kershaw (8.95).
"It's been weird," Estrada said. "I've never thought of myself as a strikeout pitcher, but for some reason, they're there, I get them. I'm not trying to strike guys out. My thought is just try to get them to swing early, get out of the inning early, and save my pitch count and get deeper into the game. If you get the strikeouts, great. If I could have zero strikeouts and pitch 200 innings with a low ERA and a lot of wins, I'm happy."
What makes Estrada a strikeout pitcher?
"That changeup," Brewers catcher Jonathan Lucroy said. "When you have a great changeup, it makes everything else better."
Estrada didn't even throw a changeup when he debuted as a professional. A pitching coach named Paul Menhart suggested in 2006 that he add the pitch to his fastball/curveball combination, but the pitch was only "OK, not great" at first, Estrada said.
The breakthrough came in 2007, when Estrada learned a new grip for the pitch from Clint Everts, a former first-round Draft pick of the Expos. In time, Estrada mastered the art of throwing changeups with the same arm speed as his fastball.
"All of a sudden, it felt really good," Estrada said. "It went from there, it got better and better, every year. Not only can that be a swing-and-miss pitch, but it makes your fastball that much better. That's big, because obviously I don't have much to work with with my fastball."
Estrada won't be working in entirely unfamiliar territory on Friday afternoon. He has experience in an opponent's home opener, having started against the Cubs at Wrigley Field last April 8, holding Chicago to two runs and five hits in seven innings. Estrada has also pitched at Fenway Park, delivering four bullpen-saving innings in June 2011 after Shaun Marcum exited after the first with an injury.
"It was loud, but it was fun," Estrada said. "There's a lot of history there, obviously, and you think about the fact so many greats have pitched there."
What will it be like seeing the Red Sox get their rings?
"It's going to stink," Estrada said. "That's obviously something every player wants in their lifetime, and for them, they just did it. It's a special moment for them. But for us, it's just another day. You say your congratulations, and that's about it. We want the same thing."