PHILADELPHIA — A 1-1 slider to Jeff Francoeur.

That was the difference Wednesday night in Rick Porcello’s Boston Red Sox debut against the Philadelphia Phillies at Citizens Bank Park. Porcello hung the pitch to Francoeur in the sixth inning, and the slugger sent it sailing over the left field fence for a three-run homer that propelled the Phillies to a 4-2 win.

“Sometimes that’s the difference — one big mistake,” Porcello said after the game. “A tight ballgame like that, I can’t make a pitch like that. I left a slider up in the middle, and he did what he was supposed to do with it.”

Porcello’s first start with the Red Sox — and his first start since signing a four-year contract extension reportedly worth $82.5 million — wasn’t perfect, but the right-hander held the Phillies scoreless until Francoeur left the yard with one out in the sixth.

On most nights, Porcello’s effort would have been enough to give the Red Sox a victory. Phillies starter Aaron Harang countered with 6 1/3 shutout innings, though, so the margin for error on a soggy night at The Bank was extremely thin.

“I thought Rick threw the ball as we anticipated,” Red Sox manager John Farrell said. “He put a number of ground balls on the infield. But credit Harang for keeping us off the scoreboard.”

It’s easy to point to Francoeur’s blast as the game-changing moment. It broke a scoreless tie and put Philadelphia in front 3-0, creating a sizable deficit on a night when runs were at a premium.

But Porcello was able to trace back his demise to two at-bats prior, when he issued a six-pitch walk to Darin Ruf before then allowing a single to Cody Asche.

“I felt good for the most part. I really didn’t think they hit too many balls hard up until that sixth inning,” Porcello said. “I think the thing before the Francoeur at-bat was the walk to Ruf. That’s a guy I’ve got to attack and make him hit the ball, make him put it in play.

“Other than that, they really didn’t hit a bunch of balls hard until that sixth inning — that Francoeur mistake.”

Porcello completed the sixth inning before handing things over to the bullpen with Boston trailing 3-0. He yielded six hits total in his three-run effort. He struck out four, walked two and threw 101 pitches (64 strikes).

The Red Sox threatened in the seventh and eighth innings, with Hanley Ramirez even coming up about three feet short of a go-ahead grand slam. But Francoeur’s homer still stood as the difference, as closer Jonathan Papelbon recorded a four-out save to give the Phillies their first win of 2015.

“I knew it was going to be up in the zone,” Porcello said, looking back on the 1-1 slider of doom. “When you make a mistake like that, you just hope that he fouls it off or misses it. But just a poorly executed pitch.”

Most of Porcello’s pitches Wednesday were good. A very bad one overrode them all.

Thumbnail photo via Eric Hartline/USA TODAY Sports Images