The Dunedin right-hander struck out Lakeland's Gustavo Nunez, allowed a home run to Ben Guez and hit Brian Holaday with a pitch. But Carreno (5-5) adjusted and fanned the final two batters of the inning on his way to a career-high 15 strikeouts, even though he eventually look the loss in Flying Tigers' 2-1 win over the Blue Jays.
"He's had a history of struggling in the first inning and we're always trying to get him through the first," Blue Jays pitching coach and 16-year Major League veteran reliever Darold Knowles said. "I thought, 'Here we go again. He's going to self-destruct.'"
The 23-year-old Dominican Republic native ended up allowing two runs on seven hits, but his 15 whiffs stood as a season high in the Class A Advanced Florida State League.
"He toughened up and pitched really well," Knowles said. "He has a really good breaking ball. It was very successful today."
Lakeland's Daniel Fields walked to begin the top of the second, but after striking out Brent Wyatt, Carreno picked the Flying Tigers' center fielder off of first base. The right-hander struck out three of the next four hitters he faced.
Carreno hit a rough patch in the fourth when Holaday led off with a double. After Billy Nowlin went down swinging, John Murrian advanced Holaday to third with a single. The Lakeland catcher ultimately scored on Daniel Fields' double to give the Flying Tigers a 2-0 lead.
"He's young and thinks too much about things like that," Knowles said. "He always pitched well as he goes along in the game. It's one of those things. With a young guy, you try to get it out of [his] mind."
Again, Carreno battled back to strike out the final two batters of the fourth before fanning two in the fifth.
Carreno found himself in trouble again in the sixth when Murrian and Fields reached on a single and double respectively. With runners on the corners, Carreno struck out Wyatt, Christopher White and Carmelo Jaime before Dunedin (48-40) replaced him in the seventh with Boomer Potts.
"It was really something," Knowles said. "I saw the lineup and knew the hitters that were coming up were susceptible to the breaking ball. I don't want to say they couldn't hit it, but some guys know it's coming and they still have trouble with it."
Dunedin's only run came on Brian Van Kirk's fifth-inning homer.
"Sometimes things go bad and you still get the win," Knowles said of the tough-luck loss. "He was okay with it. He knows that he was competitive and that's what matters."
His previous career high came when he struck out 11 Tri-City ValleyCats batters for the short-season Auburn Doubledays on July 22, 2008. He followed that with a 10-strikeout performance vs. the Jamestown Jammers one week later. Carreno has delivered two 10-strikeout performances this season (May 30 against Charlotte Stone Crabs and April 27 against the Tampa Yankees).
Before Sunday, the Clearwater Threshers' Austin Hyatt had set the Florida State League season high by striking out 14 Jupiter Hammerheads batters in a 4-0 one-hitter May 11.
Carreno, who Knowles projects as a reliever at higher levels, ranks third in the Florida State League with 118 strikeouts in 18 appearances. He has a 4.21 ERA and a 1.40 WHIP.
"His breaking ball is good," Knowles said. "His fastball is good, but his changeup is inconsistent and it's something he needs to solidify as he gets older. He's not very consistent with it yet. It's something that we have to try and master.
"I can't say he throws like anyone I've ever seen and I've seen a few of them," he added. "He always gets us late in the game and that's tremendous. He works hard and wants to get better."
Jon Kibler (2-3) got the win for Lakeland (43-44) after allowing a run on five hits over 6 2/3 innings.