BOSTON — After decisive wins over the Boston Red Sox on Saturday and Sunday, the Baltimore Orioles gave one away Monday at Fenway Park.

The Red Sox took advantage of three errors and unusually wild pitching from Wei-Yin Chen, beating the Orioles 7-1 in a game shortened to 61/2 innings because of rain.

And the expected parity is showing early in the American League East.

Boston (8-5) regained sole possession of first place, a game ahead of Baltimore (7-6). Every team in the division is within two games.

“I imagine we’re going to be neck-and-neck with everyone in this division throughout,” said Manager John Farrell. “I like the way we rebounded after dropping the last couple.”

Justin Masterson (2-0) held the Orioles to one run on three hits and three walks over five innings, striking out six.

Chen (0-1) also gave up only three hits – as well as no earned runs – but he walked four and hit a batter. He had not walked that many since a September start in 2013.

The wildness and errors doomed the Orioles.

Mookie Betts singled in the first inning. He stole second and advanced to third on catcher Ryan Lavarnway’s throwing error. Betts scored on David Ortiz’s sacrifice fly.

Betts made a mistake in center field in the second, taking a bad angle on Travis Snider’s grounder into right-center. The ball skipped by Betts and Snider got a triple. He easily scored on Ryan Flaherty’s double.

In the third, Boston scored four runs on one hit, three walks and two errors.

With two on, Chen bobbled Betts’ comebacker, allowing Xander Bogaerts to score. With two outs, Chen walked in a runner. Then third baseman Manny Machado misplayed a grounder, allowing two more runs and a 5-1 Boston lead.

“The errors were kind of gift-wrapped, along with some walks,” Farrell said.

Masterson quickly retired six of the seven batters in the fourth and fifth innings, making it an official game – something in doubt when the rain began.

“Rain aside, I’m just going out there and trying to pitch,” Masterson said. “Let’s make this quick, but also trying to compete and not just laying balls in there.”

Relievers Junichi Tazawa (one hit, 11/3 innings) and Robbie Ross (two-thirds of an inning) followed Masterson.

Orioles reliever Jason Garcia gave up two runs in the sixth, on RBI hits from Dustin Pedroia and Allen Craig.

After the top of the seventh, the umpires called for the tarp on the field and play never resumed.

The Red Sox begin a three-game series Tuesday night at Tampa Bay before going to Baltimore next weekend.

NOTES: Flaherty, the Portland native in his fourth season with the Orioles, continued his love affair with Fenway Park, going 2 for 3 on Monday and 6 for 11 in the series. In 17 career games at Fenway, Flaherty is batting .400 (24 of 60). … Hanley Ramirez left the game after two innings because of dizziness. Farrell said he expects Ramirez to play Tuesday. … Utility player Brock Holt got his third straight start at a different position (center on Saturday, second base Sunday and third on Monday), going 0 for 2 and dropping his average to .462 (12 for 26). … Lavarnway, the former catcher for the Sea Dogs and Red Sox, played his third game for the Orioles, going 0 for 3. … Garcia also used to be with the Red Sox but was grabbed by Houston in the Rule V draft and traded to Baltimore.