ST. LOUIS — With no hard evidence or even any real signs, Cubs setup man Pedro Strop leaned on his trademark optimism.

Why should anyone believe this creaky, leaky Cubs bullpen will suddenly tighten into a game-closing force down the stretch, Strop was asked Tuesday as he went on the injured list because of neck tightness.

“We’re going to be able to do this,” said Strop, who actually designs the “Believe” line of clothing, and wears the word tattooed on his arm. “Because we’ve got great guys back there; we’ve got veteran guys; and we’ve got guys with good stuff, guys that know what they’re doing. And along with the starting rotation, we’ve been throwing the ball really well.”

They’re going to need all the “Believe” mojo Strop can muster.

Because whether Strop’s right about the bullpen doing its job the last two months, it figures to be the difference between whether the Cubs make the playoffs for the fifth consecutive year.

Even if he’s right, the relief corps will have to do it the hard way.

To that end, the Cubs traded for another reliever Tuesday, acquiring surgically repaired right-hander David Phelps plus cash from the Blue Jays for pitching prospect Thomas Hatch.

Phelps has shown promise in 17 appearances (3.63 ERA) since being on the disabled and injured lists for 22 months while recovering from multiple surgeries.

He’s also the second reliever in five days the Cubs have acquired from a team willing to pay them to close the deal (also lefty Derek Holland from the Giants, who had designated Holland for assignment).

Even their signature acquisition this season, the $43 million signing of seven-time All-Star closer Craig Kimbrel last month, is a move that has required patience with Kimbrel’s inconsistent velocity and command.

Whatever the front office is able to add in the final hours before Wednesday afternoon’s trade deadline, it looks increasingly like the key to the Cubs locking down late-inning leads will be almost all about Kimbrel getting on a roll and struggling holdovers like Strop rediscovering past success.

Until an early season hamstring injury this season, Strop had been the most consistent setup man in the league since the Cubs acquired him from the Orioles in 2013.

“Listen, he’s so important to us, what he’s done over the last several years, what he’s still going to do the rest of this season, that we just want to make sure that he’s right,” manager Joe Maddon said. “So take this little time to get the neck right and get everything else straightened out. I really think it’s going to bode him and us extremely well.”

Strop, who missed a month because of the hamstring, has recently lost 2-3 mph off his fastball but said a mechanics slump that has cost movement on his sinker is the bigger issue.

“Pitching-wise I think we’re going to be all right,” Strop said, adding that the anticipated return of starter Cole Hamels from an oblique injury this weekend will be huge.

Hamels, who had a 1.22 ERA in June when the injury forced him from a June 28 start, returns to the rotation Saturday at Wrigley Field against the Brewers barring a setback.

“Hopefully, everything just gets back on track, the way we always do,” Strop said.

“It’s not how you start, it’s how you finish.”