HOUSTON -- While shaking another uneven performance in his final spring training outing last week, Houston Astros right-hander Lance McCullers exuded a confidence beyond his years, a belief that underscored his undeniable capability.

Backed by two home runs, McCullers pitched the Houston Astros to a 2-1 victory over the Seattle Mariners on Tuesday night at Minute Maid Park.

While McCullers (1-0) worked six strong innings, catcher Brian McCann and first baseman Marwin Gonzalez delivered opposite-field home runs off Mariners right-hander Hisashi Iwakuma (0-1) to provide the Astros all the offense they would need.

It marked a second consecutive game where Houston struck for two solo homers against a Mariners starter, with George Springer and Carlos Correa accomplishing the feat in the series opener against right-hander Felix Hernandez.

"We're facing pretty good pitching," Astros manager A.J. Hinch said. "It's not a foregone conclusion you can score in this league, but we did a pretty good job of fighting through what I thought was a well-pitched game on both sides.

"Iwakuma has given us trouble up and down the order, even the guys that are new, with his secondary pitches. It took two big swings to the opposite field from B-Mac and Marwin."

McCullers, 23, expressed excitement over being healthy enough to make his first Opening Day roster and take his place in the rotation after elbow problems cost him the final two months of last season. His 2017 debut went according to script.

With five strikeouts over his first three innings, McCullers faced just one batter over the minimum. He ran into trouble in the fourth, surrendering a leadoff double to Mariners right fielder Mitch Haniger to ignite a 21-pitch frame. Haniger eventually scored the tying on an infield single by first baseman Danny Valencia.

McCullers recovered to elude a bases-loaded jam by inducing a ground-ball out from catcher Mike Zunino and worked around two baserunners in both the fifth and sixth. His final line didn't fully reflect the trouble he sidestepped. McCullers allowed five hits and issued two walks (one intentional) with seven strikeouts.

"I think my command, I have made some steps in the right direction with that as far as to just being able to command my heater early versus just throwing it in the zone, throwing it different quadrants, stuff like that," McCullers said.

Said Haniger of McCullers: "He was mixing it a lot. He was keeping guys off balance. It was working for him a little bit. We just missed some opportunities with runners on base."

Seattle stranded 10 and finished 1-for-10 with runners in scoring position.

McCann, batting left-handed, took Iwakuma deep with one out in the third, depositing a 1-1 pitch into the Crawford Boxes in left field. The switch-hitting Gonzalez did the same leading off the sixth, drilling a 0-1 pitch to left field.

Iwakuma was solid, allowing two runs on four hits and three walks against two strikeouts in six innings.

"He's a pro," Mariners manager Scott Servais said of Iwakuma. "I think everyone wants to doubt him all the time, but it was spring training. You're getting ready for the season. I was excited to see him throw the ball the way he did tonight. He used all his pitches. The two home runs hurt but I thought he threw the ball great. He was very efficient. It was exactly what we needed."

The Mariners proved unable to rally against the Houston bullpen, with right-handers Will Harris, Luke Gregerson and Ken Giles preserving the one-run lead. Giles earned his second save of the season.

NOTES: Seattle moved LHP Drew Smyly to the 60-day disabled list and claimed RHP Evan Marshall off waivers from the Arizona Diamondbacks. Smyly is expected to be sidelined six to eight weeks with a flexor strain. Marshall was 0-1 with an 8.80 ERA over 15 appearances last season. ... Astros OF/DH Carlos Beltran made his first start in left field. Houston manager A.J. Hinch said during spring training Beltran would start 30-40 home games in left but backtracked a bit on Tuesday, saying 40 games "might be aggressive." With Beltran in left, Nori Aoki was on the bench while Evan Gattis, who will make his first start behind the plate on Wednesday, was the designated hitter. ... Mariners RHP Felix Hernandez underwent treatment for his groin after departing his start early on Monday night with groin tightness. While manager Scott Servais remains hopeful that Hernandez won't miss a turn in the rotation, he described the situation as "wait and see."