TORONTO -- When J.A. Happ signed with the Blue Jays in the offseason, not many would have expected the veteran left-hander to be among the best starters in the American League this season. But Happ continued his career-best year and held the Mariners to just a single on Sunday afternoon in the Blue Jays' 2-0 victory.

The 33-year-old battled control problems in the early going, and while he wasn't at his best -- walking four batters while hitting one -- the southpaw earned his career-high 13th victory. Happ's initial stint with Toronto lasted parts of three years before he was dealt to the Mariners after the 2014 season for outfielder Michael Saunders . The Blue Jays always liked what they saw from Happ, and their three-year, $36 million deal with him in the offseason has certainly paid dividends to this point.

"He's got 13 wins now and there's two months left," Blue Jays manager John Gibbons said. "There's no telling how many he can chalk up. We always liked him. We traded him because we liked the guy we were getting in return, then we made a nice investment in him to bring him back.

"All over baseball, there probably hasn't been a more consistent starter out there. He's always liked it here. He's comfortable, he's secure now and really, I think he's a better pitcher than he's ever been."

The numbers have backed that up this season. During Happ's first stint in Toronto, the lefty never had an ERA lower than 4.22. His outing on Sunday dropped his ERA to 3.27, and he held his opposition to one run or less for the 10th time in his 20 outings.

The former Phillies top prospect threw only 54 of his 103 pitches for strikes, but he also managed to keep the Mariners at bay during a crucial fourth inning. After putting the first two hitters on base, Happ struck out Nelson Cruz and Dae-Ho Lee with 93-mph fastballs before inducing a popup off the bat of Kyle Seager .

"Yeah, that's kind of my best pitch, so I tried to use that and get ahead with that," Happ said about using his fastball in the fourth inning. "When you're not hitting on your other stuff, you have to rely on that. I was just trying to really locate there, and mixing it up with the changeup helped."

Part of Happ's improvement, according to Gibbons, has also been an increased level of maturity and confidence. The Blue Jays' starting rotation has pitched a Major League-leading 63 quality starts this year, and Happ's 13 have certainly provided a boost in Toronto's chase for another American League East crown.

"He's having a tremendous year," Gibbons said "He's a really different pitcher than he used to be. I thought he was off early in the game, and then you look up and he's only given up a hit. I thought he got better as the game went on. "

An added bonus for Happ was that the victory came against his former team. Happ struggled with the Mariners in 2015 before getting traded to the Pirates midseason and turning around his campaign. Happ retired nine of the last 10 hitters he faced to pick up the victory in his first game against his old teammates.

"You try to put that out of your mind, but after being in the other dugout for four months it is a little different facing those guys," Happ said. "That's kind of the way it is"

Alykhan Ravjiani is a reporter for MLB.com based in Toronto.