It was deja vu all over again for the Red Sox bullpen.

Handed a 4-3 lead and the chance to pick up a huge series-opening win over a Houston Astros team that couldn’t stop shooting itself in the foot, the bullpen once again let things slip away in heartbreaking fashion.

This time it was Lucas Sims’ turn to blow the lead, and the trade deadline acquisition allowed back-to-back singles with two outs in the eighth. That set the stage for the bottom of the ninth, when Yainer Diaz took Kenley Jansen deep for a towering walk-off home run to beat the Red Sox 5-4.

It was Boston’s 12th blown save since the All-Star break, and now the Red Sox find themselves 4.5 games back of the Kansas City Royals for the last American League Wild Card spot.

“Sims has done a good job and he had a good lane there, it just didn’t work out,” manager Alex Cora said.

The Red Sox also squandered one of Tanner Houck’s best starts in over two months, along with perhaps the worst defensive game the Astros have played all season.

Jarren Duran led off the game with a first-pitch leadoff home run, and Boston tacked on a second run thanks to an error by Astros third baseman Sean Whitcomb. It wound up being the first of four errors committed by the Astros, the most in a single game by Houston this season.

The Red Sox didn’t play a clean game either, and their big error in the bottom of the fourth proved costly. After the first two batters reached on singles, Rafael Devers was able to gun down the lead runner at the plate on a soft grounder by Jeremy Peña. But whatever momentum Boston got on that play was quickly squandered when Victor Caratini tied the game on a play that the Red Sox immediately wished they’d gotten back.

Caratini’s single to right was fielded by Tyler O’Neill, whose throw to the plate was well off the line. As a run crossed the plate catcher Danny Jansen tried to catch Caratini off the first base bag, but his throw sailed into right field, allowing Peña to come in to tie the game at 2-2.

Houston repaid the favor, however, in the fifth and sixth innings when a couple of routine plays went awry. Diaz, a catcher who was playing just his 10th MLB game at first base, was pulled off the bag in both instances. The first came with two outs in the fifth and led to a first-and-third scoring opportunity that the Red Sox couldn’t capitalize on. The second led off the sixth and set the stage for Masataka Yoshida’s pinch hit two-run home run.

Yoshida, who came off the bench with the lefty Yusei Kikuchi on the mound for Houston to start, has been on an impressive heater recently. Over his last 42 games entering Monday dating back to June 24 Yoshida has been batting .351 with a .958 OPS, and with his home run he’s now riding a seven-game hitting streak. He’s been outstanding as a pinch hitter too: in seven plate appearances he’s now 3 for 5 with a homer, two doubles and two hit by pitches.

“He’s locked in,” Cora said. “This is the guy we saw when we signed him.”

That home run made it a 4-2 Red Sox lead, and thanks to Romy Gonzalez’s great off-balance throw from behind second base to end the fifth inning, Houck entered the sixth with a chance to polish off his 18th quality start of the season. He allowed two singles to start the inning and a sacrifice fly by Jon Singleton, but was able to finish his outing strong. His final three pitches each approached 97 mph, the last of which drew a lineout by Mauricio Dubon.

Houck’s final line: three runs (two earned) on eight hits over six innings with eight strikeouts and no walks. It was the first time Houck hadn’t issued any walks since June 13.

Unfortunately Houck didn’t factor into the decision. Lucas Sims came on for the eighth, got another defensive gem from Gonzalez to turn a double play but proceeded to allow back-to-back singles to tie the game at 4-4. Chas McCormick had the first and came around to score on Dubon’s go-ahead RBI knock.

Boston couldn’t respond in the top of the ninth, and after Jansen struck out Alvarez on three straight pitches, Diaz only needed one to finish the Red Sox off.

Weissert called up

Prior to the game the Red Sox made a switch in their bullpen, calling up right-hander Greg Weissert while optioning left-hander Bailey Horn to Triple-A Worcester.

Weissert was one of the club’s most reliable bullpen arms through the first half but was among those whose production fell off dramatically in July. Weissert posted a 2.65 ERA as of June 25, but over his last 11 MLB appearances between June 28 and July 28 he allowed an 8.53 ERA, raising his total for the season to 4.24.

Since going to the minors he’s appeared in four Triple-A games, and in those outings he’s allowed one earned run over six innings (1.50 ERA).

Horn has appeared in 14 big league games for the Red Sox this season, posting a 6.48 ERA in 16.2 innings. He appeared in three of Boston’s four games against the Orioles this past weekend.