By earning their fourth victory in six games, the Angels kept Boston from gaining ground in the American League East. The Red Sox remain in third place but fell two games behind the new leaders, the Toronto Blue Jays.
Left-hander Hector Santiago won his sixth consecutive start despite throwing a season-high 119 pitches during his five innings. Santiago (10-4) allowed two runs, four hits, six walks and a wild pitch but finished with six strikeouts. Closer Huston Street pitched a perfect ninth inning with one strikeout for his ninth save.
Mookie Betts hit his 21st home run but the Red Sox left 11 runners on base in sustaining their fifth loss in six games.
Left-hander Drew Pomeranz lost for the second time in as many decisions since being acquired from the San Diego Padres. In 5 1/3 innings, Pomeranz (0-2) permitted five runs, six hits, two walks and a wild pitch while collecting four strikeouts.
The Red Sox held a 2-1 lead when Pujols hit his 20th homer of the season and 580th of his career. One out after Yunel Escobar walked, Pujols propelled left-hander Drew Pomeranz's first pitch -- an 87 mph cutter -- over the fence in left-center field.
Pujols became the fourth player in major league history to amass at least 20 home runs in 15 of his first 16 seasons. The others? Hall of Famers Hank Aaron, Willie Mays and Frank Robinson.
The Angels' designated hitter also tied Hall of Famer Lou Gehrig for ninth place in career extra-base hits with 1,190.
The Red Sox sent eight batters to the plate in the first inning and took a 2-0 lead.
Betts began the game by hitting Santiago's third pitch, an 89 mph fastball, into Boston's bullpen behind the left-field fence.
The drive elicited loud cheers from the substantial number of Boston fans, who chanted "Let's Go Red Sox" throughout the game.
Santiago walked Dustin Pedroia and Xander Bogaerts before striking out Jackie Bradley Jr. But Hanley Ramirez followed with a single just past third baseman Escobar that brought Pedroia home.
Travis Shaw walked with two out to load the bases but Santiago defused the threat by making Bryce Brentz look at a called third strike.
The left-hander threw 38 pitches in the first inning, divided evenly between balls and strikes.
The Angels responded in the bottom of the first by narrowing their deficit to 2-1 and putting the potential tying run in scoring position against Pomeranz, making his third start since being acquired from San Diego on July 14.
With one out, Mike Trout lined a single past just past Bogaerts's glove at shortstop, then Pujols followed by dumping a single into short center field.
Both advanced into scoring position on a wild pitch and Trout scored when Jefry Marte grounded out.
Boston again loaded the bases with two out in the third. Bogaerts singled before Bradley and Shaw walked. But Brentz again looked at a called third strike, this time on a 94 mph fastball, to end the inning.
The Angels expanded their lead to 4-2 in the fourth. Andrelton Simmons doubled down the left-field line, took third on Jett Bandy's long fly out and scored on Johnny Giavotella's single past drawn-in third baseman Aaron Hill.
NOTES: Boston 2B Dustin Pedroia needs one extra-base hit to move past Nomar Garciaparra and take 10th place in the team's career list. ... Red Sox RHP Craig Kimbrel began a rehabilitation assignment with Triple-A Pawtucket. He underwent knee surgery on July 11. ... Boston DH David Ortiz, who was not in the starting lineup, needs one hit to tie Hall of Famer Mickey Mantle for 20th place all time with 2,415. Ortiz also needs to score one run to move past Hall of Famer Joe DiMaggio and move into 95th place with 1,391. ... Los Angeles activated OF Shane Robinson from the disabled list and designated OFs Daniel Nava and Craig Gentry for assignment. Robinson batted ninth and played right field for OF Kole Calhoun, who was not in the starting lineup.
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