SAN FRANCISCO – The Cubs opened the second half by sweeping the Pirates to strike a blow to Pittsburgh’s hopes of hanging with the big boys in the NL Central.

Then they beat the Reds in a series for the first time to deal a painful blow to their hopes.

But when it comes to striking a Blow by the Bay to further their goals at next week’s trade deadline, the Cubs exposed their roster needs instead.

One night after losing when struggling Pedro Strop blew an eighth-inning lead, the Cubs’ inability to hit left-handed pitching most of Tuesday night helped send them to a second 5-4 loss in as many nights – this time on Pablo Sandoval’s one-out walkoff homer in the 13th inning.

So much for forcing the fence-sitting Giants’ hand into selling at the deadline. So much for motivating the Giants to talk about trading back-end lefty bullpen studs Will Smith or Tony Watson to the Cubs.

And forget about Tuesday night being World Series hero Madison Bumgarner’s last home start as a Giant.

The loss sent the Cubs to their eighth consecutive winless series on the road, crushed their wobbly bullpen and underscored a flawed roster that could use an impact player but needs at least more depth.

And they’re not about to get any help from the Giants.

As much as Giants team president Farhan Zaidi looked a month ago like a certain seller at the deadline – and as much as he might privately yearn to sell from his flawed roster for young assets – the players on the hottest streak in the National League are making that all but impossible.

“I would bet a month ago it was an easy thought,” Cubs manager Joe Maddon said as the series opened Monday. “Probably right now it’s not so easy.”

In iconic manager Bruce Bochy’s final season before retirement, the Giants suddenly have 17 wins in their last 20 games – the hottest team in the league padding their winning record and closing in on playoff position.

“There’s a tangible feeling around the locker room of guys trying to play for [Bochy] and trying to send him out the right way,” said Giants starter Jeff Samardzija, the former Cub. “And understanding that if we’re in it and having the leverage of ‘Boch’ [in his final year], you would assume that we probably help the team out [at the deadline] instead of subtracting from the team.”

If anything, the Giants this week are an in-your-face reminder of how difficult it might be to acquire the slam-dunk, late-inning lefty the Cubs desire – especially with limited resources both in terms of payroll flexibility as well as player capital they’re willing to part with.

The Giants (Watson, Smith), Indians (Brad Hand) and Nationals (Sean Doolittle) all have played well and into playoff contention since projecting early last month as sellers with desirable left-handed closers and setup men.

As team president Theo Epstein alluded to during the recent homestand – and one rival executive surmised – the Cubs might wind up with more depth moves, on the bench and in the bullpen, than impact moves.

The Cubs have been linked to Blue Jays infielder Eric Sogard as an answer to their second base deficit at the plate and Detroit outfielder/corner infielder Nicholas Castellanos as an answer to their inability this season to hit left-handers.

That came into play again Tuesday when the Cubs went 7-for-34 (.206) against lefties Bumgarner, Watson and Smith.

The real difference might have to come with bullpen help.

“Of course, you could always use more bullpen depth; I’ll never deny that because I’ve always believed that,” Maddon said after Strop’s blown save Monday – the 17th in 41 chances for the bullpen this year. “That’s a game we should have won. Pedro will be the first guy to tell you that, too. Yes, you always want more bullpen, yes.”

But he said he trusts Strop’s track record, and thinks he’ll be OK. And he’s optimistic about Edwards returning to competitive form.

But they need a lefty.

Their impact move may already be in house in closer Craig Kimbrel, signed last month. Getting him the ball with a ninth-inning lead often enough for that impact is the challenge now.