As large as his shadow is, Buck Showalter was not the biggest absence Wednesday night.

The Mets also were missing the starting pitcher who had twirled two gems and an offense that has powered the fast start with timely hits.

Both vanished against the Giants, with Chris Bassitt knocked around early and the bats overmatched by Carlos Rodon in a 5-2 Giants victory that snapped the Mets’ three-game winning streak in front of 30,050 at Citi Field.

Presuming Showalter, who missed the game because of a medical procedure, returns Thursday as expected, the Mets (9-4) will finish 0-1 without him. With Showalter out, a manager-by-committee that featured hitting coach Eric Chavez, pitching coach Jeremy Hefner and player development coordinator Dick Scott took the reins.

“Obviously we missed Buck and his leadership, but [I] thought, given the circumstances, we did a good job,” said Hefner, whose staff was active and pinch-hit several times in the later innings.

Perhaps Showalter was missed in the seventh inning, when Starling Marte ran into an out on an attempted steal with the Mets down four. But Marte said he always has the green light.

The Mets managed without Showalter, but they could not manage without the Bassitt who had allowed one run in his first 12 innings this season. Against a Giants offense that was mostly tamed by Mets pitching over 19 innings on Tuesday, Bassitt was in trouble immediately.

Five of the first six Giants hitters reached base, and before many fans had reached their seats, the Mets were in a 3-0 hole. The Giants kept finding holes with a single, a walk, a double and two more singles to provide all the runs they would need all night. The Giants could have scored another, but Pete Alonso grabbed a Thairo Estrada safety squeeze and came home for the out.

“They had a really good plan and executed really well,” said Bassitt after his first unspectacular start of the season.

Several of the hits were not hard-hit, which was no consolation.

“This one won’t feel good. I’m not going to take any positives from this one,” Bassitt said.

After Bassitt threw 25 pitches in the first inning, Brandon Belt golfed a home run into the upper deck in right field off him in the second. Bassitt settled down from there and lasted six innings, in which he allowed those four runs on eight hits and a walk. He got stronger as the game got longer, striking out the side in the fifth and providing length after a shaky opening, just as Tylor Megill had done Tuesday.

The reigning NL West champion Giants entered play as the only team with a better ERA than the Mets. After the Mets handled San Francisco’s co-ace, Logan Webb, on Tuesday, they had issues with the other front-of-the-rotation stud.

Rodon was dominant over five innings, in which he struck out eight and surrendered three hits and two walks. The Mets had a few opportunities they could not cash in, going 0-for-4 against Rodon with runners in scoring position. Numbers 7-9 in the Mets’ lineup — often a strength over the first two weeks — went 0-for-10 with five strikeouts with two walks.

After No. 9 hitter James McCann whiffed in the seventh inning, there were boos heard for the catcher, who is 3-for-25 (.120) through his first nine games of the season.

The Mets avoided their first shutout of the year by getting to the San Francisco bullpen in the seventh inning, when Marte blooped a single between three Giants defenders that scored Luis Guillorme. There were runners on the corners for Francisco Lindor, but with the Mets down four runs, Marte tried to swipe second and was gunned out for the final out of the inning.

The Mets added another run in the eighth with a Mark Canha RBI single, but they stranded two more when pinch hitter Robinson Cano grounded out and Dominic Smith, also pinch hitting, lined out to old pal Wilmer Flores at third base.

“We were home run away from this being a really close game,” said Hefner, who couldn’t find that home run.

The Mets were missing a big piece from their dugout and missing a big hit, leaving nine on base.