"Game-changer," Angels teammate Chris Iannetta said. "It was awesome."
Seven innings later, Iannetta delivered the tiebreaker with an RBI double that sent Los Angeles over the Boston Red Sox 4-3 on Tuesday night.
Brennan Boesch had a ground-rule double with two outs off closer Koji Uehara (5-3) for the Angels' first hit since the fourth. Iannetta then hit the ball off the end of his bat off the left-field wall, just above the leap of Daniel Nava.
But Calhoun's grab in front of his team's bullpen was the major topic in the Angels' clubhouse.
"We go from being potentially down three or four runs to having the opportunity to take the lead," Iannetta said.
Down 1-0 on David Ortiz's 29th homer in the first, the Angels went ahead with three runs in the third.
"This game's a lot about momentum swings," Calhoun said, "and that's a big one right there."
Joe Smith (5-1) got the win for the AL West leaders and Huston Street recorded his 34th save.
Boston rookie center fielder Mookie Betts, a second baseman in the minors, also took away a potential home run when he ran down Josh Hamilton's drive in the first in deep right-center field.
"Every game played in center field is going to be a positive experience for him, given that he's gone through a positional change," Red Sox manager John Farrell said.
The Angels took a 3-1 lead with four consecutive hits, starting with a double by Iannetta. The next three hits all drove in runs - a single by Calhoun, a triple by Mike Trout and a single by Albert Pujols.
Pujols originally was ruled out for the second out of the inning on Will Middlebrooks' throw after the third baseman's diving backhand stop. The call was overturned on replay, but Trout would have scored anyway on his headfirst slide into home.
Jered Weaver's wildness led to Boston's next two runs that tied it at 3 after six innings.
Xander Bogaerts began the sixth with a walk, took third on a single by Christian Vazquez and scored on a sacrifice fly by Holt.
Angels: LHP Joe Thatcher isn't expected to return from the disabled list as hoped by Friday for the start of a three-game series with Oakland. He has been sidelined since Aug. 3 with a sprained left ankle.
Red Sox: Middlebrooks left the game in the fourth with right hamstring tightness. Kelly Johnson pinch-ran after Middlebrooks was hurt while beating out a grounder to shortstop for a single. ... C David Ross is expected to be activated Wednesday after going on the disabled list with right plantar fasciitis.
Angels: RHP Garrett Richards (13-4, 2.53) pitches in the third game of the series Wednesday night. He won his last three games, including a complete-game shutout against the Los Angeles Dodgers and a 5-4 victory over the Red Sox.
Red Sox: RHP Clay Buchholz (5-7, 5.79) has a 3.00 ERA in his last two starts after a 9.00 ERA in his previous four, a stretch that followed an 11-0 complete-game win over Houston.
With a runner at second and two outs in the ninth, Street walked Ortiz intentionally then struck out Napoli. "Both guys are going to hit a mistake and Huston didn't make any," Angels manager Mike Scioscia said. Ortiz leads the majors with 91 RBIs.
Red Sox rookie starter Allen Webster pitched six solid innings, retiring his last five batters and allowing runs in just one inning. He's in the regular rotation after Boston traded four starters last month. "I feel pretty confident right now on the mound," he said.
Josh Hamilton went 0 for 3 and was hit by a pitch after sitting out two games following a 10-game stretch in which he went 5 for 38 with 18 strikeouts. He took extra work before the game with hitting coaches Don Baylor and Dave Hansen. "He's going to have to figure it out," Scioscia said. "He's not going to figure it out sitting."