SAN FRANCISCO -- One popular theory explaining how the Cubs can pry a much-needed left-handed reliever from the Giants, who rallied past them Monday night, 5-4, at Oracle Park, featured a simple strategy.

According to this plan, the Cubs must sweep the Giants in this three-game series, thus dampening San Francisco’s National League Wild Card hopes. Let the Giants sink beneath two or three more teams in the Wild Card standings, and they’ll have no choice but to part with one of their talented lefties by the July 31 Trade Deadline.

This theory was invalidated when the Giants scored three runs in the eighth inning to erase a 4-2 Cubs lead. Still, the Giants are reportedly considering any and all trade proposals, so maybe Cubs management can engineer a deal.

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Maybe the Cubs could get Drew Pomeranz, the erstwhile starter who struck out four in two perfect innings of relief Monday.

Maybe the Cubs could acquire Tony Watson, who has struggled this year against lefties -- they’re batting .333 off him -- but possesses the savvy to excel down the stretch in any role.

Or maybe the Cubs even could obtain All-Star closer Will Smith, who could handle setup duties if Craig Kimbrel were to remain Chicago’s closer.

The Cubs certainly could have used at least one more left-handed reliever against the Giants, who surged above the .500 mark (51-50) with their ninth victory in 10 games.

Chicago’s sole lefty in the bullpen, Kyle Ryan, was out of the game, having pitched 1 1/3 innings of shutout relief, when the Giants rallied in the eighth. Their uprising started with a double by switch-hitter Pablo Sandoval, who’s significantly weaker from the right side. Using a lefty against Sandoval might have stopped the rally before it started.

Later in the eighth, left-handed-batting Joe Panik contributed the go-ahead RBI with a single. Panik probably would have been removed for a pinch-hitter if the Cubs had a lefty available.

Fantasy, however, is not a strategy. Cubs manager Joe Maddon must make the best of his roster as it is constituted.

“Of course, you can always use more bullpen depth,” Maddon said. “But I felt really good about the way we played tonight.”

In fact, the Cubs jumped ahead, 2-0, on homers by Robel Garcia in the second inning and Kyle Schwarber in the third. They nursed that two-run lead into the eighth and entrusted it to Pedro Strop, who performed far from the standard he has maintained for most of an 11-year career.

Sandoval doubled leading off and scored on Brandon Crawford’s one-out ground-ball single. Kevin Pillar’s forceout left Strop one out away from escaping, but Austin Slater doubled to center, tying the score.

“I wanted a sinker in,” said Strop, whose pitch caught too much of the strike zone. “I have to keep working on it and find a way to make it sink.”

Panik’s hit, a sharp grounder past third base, put the Giants ahead and left the Cubs hoping for better nights.

“Strophy is really able to move things forward, but he’ll review. ... He’s been there before,” Maddon said. “He’s been one of the best relievers in baseball for years for a reason, and I think one of the biggest reasons is that he’s able to take a bad moment and throw it in the trash can fast.”