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SAN FRANCISCO -- The Giants welcomed back two key pieces of their lineup on Monday, when outfielders Mitch Haniger and Austin Slater were activated off the injured list after missing the first three weeks of the regular season.

Haniger, a Bay Area native who signed a three-year, $43.5 million deal with his hometown club over the offseason, played left field and batted third in his Giants debut, while Slater slid into the leadoff spot and started at designated hitter.

The two right-handed hitters are expected to help beef up San Francisco’s lineup against left-handed pitchers, who have held the Giants to a paltry .597 OPS this season. With Haniger and Slater back in the fold, the Giants were able to bat Wilmer Flores sixth in their 4-0 win on Monday, helping to lengthen the lineup against Cardinals lefty Jordan Montgomery.

“It’s a little thing,” manager Gabe Kapler said before the game. “Up and down our lineup we’ve got quality bats that can take a walk and hit a home run and give us a good look against a tough left-handed pitcher in Montgomery."

Montgomery worked six scoreless innings before the Giants finally chased him with a four-run seventh that was highlighted by Haniger’s sacrifice fly and J.D. Davis’ three-run shot to left field. Slater went 2-for-3 with a pair of singles, while Haniger finished 0-for-2 with a walk and an RBI, his first for the Giants.

“If Slates keeps doing that, then we’ll be right where we need to be,” said Alex Cobb, who stole the show by firing his second career shutout. “Hanny is such a presence in the lineup. I faced him a lot when I was with [the Angels]. He’s one of those guys that when you’re going over the scouting report, you spend a lot of extra time on. He’s just a very disciplined player all around.

“I think that obviously we haven’t played as well as we want, but now is the time that we’re starting to get guys back and we’re getting our full team. We’ll push each other to try to get back to the place where we feel like we need to be.”

Haniger, 32, was the Giants’ biggest offseason investment and crushed 39 home runs in his last full season with the Mariners in 2021, but he strained his left oblique during Spring Training and then experienced back tightness while ramping up earlier this month. Despite the setbacks, Haniger looked dialed in at the plate during his rehab assignment with Triple-A Sacramento, homering twice in his four games with the River Cats.

“We have a good team here,” Haniger said. “We’re starting to get healthy. That’s the most important thing. I’m just excited to be back and help this team win. When we’re healthy, we’re going to have a really good team. We’re getting there, so it’s good to be back.”

Haniger and fellow free-agent addition Michael Conforto are expected to serve as the Giants’ everyday corner outfielders, though the two still seem to be getting their bearings at Oracle Park. Haniger, who played left field here for the first time on Monday, appeared to miscommunicate with center fielder Mike Yastrzemski in the eighth, when he pulled up at the warning track and allowed Lars Nootbaar’s fly ball to drop for a double.

Slater is expected to form a platoon with Yastrzemski in center this year, but he wasn’t quite ready to return to the outfield due to a right elbow issue that sidelined him this spring and continued to bother him while he was working his way back from a left hamstring strain.

Slater was limited to DH duties when his elbow flared up in the second half of 2020, but Kapler said he believes the 30-year-old is slowly building up and should be able to handle the outfield soon.

“To be frank, Austin’s comfort level will dictate our comfort level,” Kapler said. “The more confident he is to let it rip, the more confident we’re going to be in getting him out there as often as possible.”