That is when the Indians pitcher blew out his right knee.
Talk about an embarrassing phone call to the Indians' medical staff.
"It was," Kluber said this week. "My knee was stuck back, because the meniscus flipped under and got pinched between the bones. I couldn't straighten my leg out. I was basically lying down on the couch trying to straighten it, telling them, 'Hey, I don't really know what happened.'"
It is a funny story to tell because of what has happened in the months since the injury.
Kluber heads into his Friday start against the Rangers as one of the more pleasant surprises this season for a Cleveland club that is trying to make a run at the American League Central crown. The rotation has played an integral role in the Tribe's turnaround from last year, and Kluber has emerged into a reliable cog within the cast.
When Indians manager Terry Francona first watched Kluber in the spring, the right-hander was still in the final stages of his knee rehab, which was apparent by the on-field results. When Francona last saw Kluber work, he was stringing zeros together in an outing against the Twins on Saturday at Target Field.
"I think it's almost amazing. I'm so proud of him," Francona said. "It's because of guys like him -- you've heard me refer to the glass being half-full a lot of times, and that's why. We're trying to fight it out in this division with guys like Corey and we're excited about it. That's pretty cool.
"He's getting better, it seems like, almost every start, whether he's learning something or gaining experience. And, in the meantime, he's winning games."
On the season, the 27-year-old Kluber has gone 7-5 with a 3.69 ERA across 18 appearances, including a pair of relief outings in April. Kluber's path to the rotation was paved when veteran Brett Myers -- signed to a one-year contract over the offseason -- was shelved on April 20 with a right elbow injury that has yet to fully recover.
The knee, which was surgically repaired on Nov. 3, has not been an issue for Kluber.
Kluber has more than held his own, especially since his wakeup call on May 10 in Detroit. A large part of the pitcher's success this season has been an aggressiveness with his two-seam fastball that was not there in his dozen starts as a rookie last year. Kluber strayed from that approach against the Tigers, who thanked him by scoring eight runs in 4 2/3 innings.
"That outing was something stupid -- like 30-percent fastballs," said Kluber, who still shakes his head at the performance two months later. "I'm not saying I need to throw my fastball every other pitch, but you have to throw it to make them respect the other stuff. That was a big eye-opener for me."
Since that start against the Tigers, when 33 percent of his pitches were indeed fastballs, Kluber has posted a 3.13 ERA over 13 starts, piling up 81 strikeouts against 18 walks in 77 2/3 innings.
Evidence of Kluber's evaluation of that start in Detroit can easily be found within the numbers.
The pitcher has averaged just over 100 pitches in his 15 normal starts, excluding his relief or rain-shortened games. When Kluber has logged at least 45 sinkers (roughly 45 percent), he has posted a 2.50 ERA this season within that sample. When the righty has thrown fewer than 45 two-seamers, his ERA balloons to 8.57.
Among AL starters with at least 70 innings this season, Kluber's average fastball velocity of 93 mph ranks sixth. Ahead of him are five of the league's elite arms: David Price, Derek Holland, Max Scherzer, Yu Darvish and Chris Sale.
"He knows that when he pitches aggressively with his fastball, he's a much better pitcher," Francona said. "And there's innings where he's gotten away from it, and he knows it, but that's part of maturing as a pitcher. When the damage is starting, handling it, regrouping, trusting your stuff, that's part of it."
Indians pitching coach Mickey Callaway said the roots of Kluber's transformation were planted last season.
As the Minor League pitching coordinator at the time, Callaway noticed that Kluber was too reliant on his four-seam fastball and offspeed pitches in the first half with Triple-A Columbus. Callaway discussed things with Ruben Niebla, Columbus' pitching coach at the time, and the consensus was that the pitcher could benefit from focusing more on his two-seamer.
If Kluber could establish that pitch aggressively and efficiently, it would open the door for his slider, cutter and changeup to be more effective.
Last year, Kluber posted a 6.38 ERA in May, a 3.12 ERA in June and a 2.91 ERA in July at Triple-A and was promoted to the big leagues in August.
"It was totally night and day from the first half to the second half last year," Callaway said. "Once he started throwing his two-seamer, we kind of left him alone and let him do his thing, knowing he was getting more command of his fastball. That's what helped him to get up here in the first place."
Kluber did endure some growing pains early on in his first real taste of the Majors in 2012, allowing six runs in his starting debut on Aug. 2 and finishing the season with a 5.14 ERA with the Tribe. Within the ups and downs, however, were lessons learned, which have been applied this season on the mound.
"It was a good learning experience for me," Kluber said. "I did go through some troubles at the beginning, but toward the end of the year I had some good outings to build off. I think I kind of carried that over into this year, trying to take an aggressive approach.
"I think I was getting caught up a little too much in the different scouting resources last year: 'This against this guy,' or 'This against that guy,' instead of falling back on what works for me. Stick with what you do well."
Does that include still changing diapers?
"I still do it," Kuber said with a laugh. "You've got to have that elevated changing table. You can't do it on the floor."