BOSTON -- An early three-run lead wasn’t enough for the Indians to fend off the Red Sox in Monday’s series opener at Fenway Park. Especially when, in the end, the Indians gave up a season-high 12 runs.

“They’re aggressive, they can hit,” right fielder Oscar Mercado said. “But at the same time, we definitely had the game, we had the momentum going and you can’t let that happen.”

• Box score

The Indians got on the board early against Red Sox starter Rick Porcello with two runs in the first inning, driven in by Jason Kipnis and Roberto Pérez, who has 12 RBIs this month. They tacked on another in the third, thanks to a Jake Bauers RBI groundout.

The Indians entered the game 16-6 when scoring first, but that advantage would change. After the Red Sox evened it up at 3 against Cleveland starter Jefry Rodriguez, they blew the game wide open in the fifth.

The defending World Series champions batted around and scored six runs off five hits, including three doubles and a three-run home run from Sandy Leon.

Rodriguez exited after 4 1/3 innings (8 hits, 6 earned runs, 3 walks, 6 strikeouts and 1 wild pitch) with two runners on base. He also was assessed a balk during the second at-bat of the fifth inning.

“I just wanted to speed it up compared to the previous ones,” Rodriguez said of the balk, through an interpreter. “Because on the previous ones I held it for a long time, and I just wanted to go quick on that one.”

Oliver Pérez (two-thirds of an inning, 2 hits, 2 earned runs and 1 home run) completed the game-changing inning. He noticed the Red Sox jumping on opportunities at the plate.

“I think we were leaving too many pitches around the zone with two strikes, so they took advantage of that,” Perez said. “That’s a good team that can hit any time, any count. So I think we left too many two-strike pitches around the zone. You can’t do that to any team.”

The Indians allowed multiple home runs for the 19th game this season. Mercado was unable to catch J.D. Martinez’s solo shot to right field in the sixth inning, which gave the Red Sox a 10-3 lead.

“I’m running for it and as I reach up for it, I didn’t get to it and it hit the top of the wall and came down and hit my glove in the process of me crashing into the wall,” Mercado said. “Just one of those freak things.”

The Indians’ previous season-high of runs allowed was 11 against the Braves on April 21. In the end, manager Terry Francona said both the Indians’ pitching and the Red Sox offense factored into the Indians’ seventh loss in eight games. The Indians remain winless (0-24) when trailing after the sixth inning this season.

“To the Red Sox credit, they keep coming at you once they kind of smell some blood,” Francona said. “We just didn’t have an answer that inning.”