SAN DIEGO – The Giants opted to go a man short in their bullpen to create enough flexibility to carry an extra position player on their 2024 Opening Day roster. But the relief corps dwindled even further following an untimely injury that cost them a late-inning lead in a 6-4 loss to the Padres on Thursday afternoon at Petco Park.

San Francisco took a 3-2 lead into the bottom of the seventh inning behind a quality start from ace right-hander Logan Webb and a pair of RBIs from new shortstop Nick Ahmed, but the Padres staged a four-run rally against veteran reliever Luke Jackson, who faced three batters without recording an out before departing with lower back tightness.

“I wish we could have gotten Webby the ‘W’ and gotten us off to a great start,” Jackson said. “I thought we played great baseball today. The team looks great. All the guys [who] came in after looked great. Webby looked fantastic. It was just one of those days that the ball doesn’t go to the guys, and it’s part of it. I think if we keep playing this kind of ball, we’re going to win a lot of games.”

Jackson surrendered back-to-back singles to put runners on the corners for the Padres, who went on to tie the game on an errant throw by catcher Patrick Bailey. Bailey tried to catch Tyler Wade stealing after he took off for second base, but his throw short-hopped Ahmed and ricocheted into the outfield, bringing home Luis Campusano from third.

After issuing a walk to Jackson Merrill, Jackson immediately signaled for trainer Anthony Reyes and departed the game. Ryan Walker was summoned to replace Jackson, but he couldn’t stop the bleeding, giving up a go-ahead single to Xander Bogaerts, followed by a two-run double to Jake Cronenworth.

“We take a lead going into the seventh and just couldn’t stop them,” manager Bob Melvin said. “After seven, you expect to hopefully close it out.”

Jackson, 32, missed nearly a month with a lower back strain last year, but he’s hoping he won’t have to miss as much time with his latest flare-up. He was expected to undergo an MRI exam on Thursday night to determine the severity of the injury, though his questionable status could force the Giants to add another pitcher to the roster prior to Friday night’s game.

“It doesn’t feel as bad as last year, so I’m hoping for the best and praying it’s nothing too bad,” Jackson said. “But you never know. I’m getting old these days, I guess.”

Before the tide turned in the seventh, the Giants appeared in line to open their season on a high note thanks to the effort by Webb, who delivered six innings of two-run ball in his third consecutive Opening Day for the club. Ahmed and Michael Conforto led the way offensively, with Conforto launching a solo shot in the top of the ninth to cap a three-hit day.

New center fielder Jung Hoo Lee also enjoyed a productive day in his Major League debut, lifting a sacrifice fly that briefly put the Giants ahead, 3-2, in the top of the seventh and collecting his first career hit -- a single to center field -- against Yu Darvish in the fifth. Lee, 25, didn’t have much time to process the milestone, as he was quickly picked off by Darvish to end the inning.

“I did get the hit, but right after that, I got picked off by Darvish, so I never had the time of feeling that I actually got a hit,” Lee said via interpreter Justin Han. “I’m kind of sad about that. … The level is way more high in the Major Leagues. The level of fans is at a higher volume, also.”

Webb endured a rough showing during Spring Training, logging a 10.97 ERA over six Cactus League starts, but he looked back to his elite self on Thursday, facing the minimum through the first four innings before issuing a leadoff walk to Manny Machado that set the stage for a two-run inning for the Padres in the fifth.

Even with the Giants’ upgraded defense, several Padres hitters managed to find holes up the middle against Webb, who allowed five hits while striking out five in the 97-pitch outing.

“It’s just the way it was going today,” Webb said. “Sometimes baseball is cruel like that. I thought the guys did a great job today. It’s easy to get down on yourself after the first one, but we’ve got 161 more of these, which is crazy to think about. But we’re going to show up tomorrow and try to win a game just like we did today.”