The three-time All-Star was lifted with none out in the fourth after giving up three hits, including a tiebreaking two-run homer by Chris Coghlan, and the Los Angeles Dodgers ended up with a 7-3 defeat on Sunday in the rubber game of the series.
"I've got to do a better job of keeping the team in the game, not killing our bullpen every time I pitch," said Beckett, who struck out Ryan Sweeney with the bases loaded to end the third.
"You've got to make better pitches. Those guys are big league hitters. If you don't make pitches to these guys, they get hit. I didn't throw enough strikes, and the strikes I did throw weren't good enough."
Beckett (6-6) is 3-5 with a 3.30 ERA in 10 starts since his no-hitter on May 25 at Philadelphia, and has surrendered five homers in his first three starts off the disabled list after missing two weeks with a sore hip. One of his biggest problems, according to manager Don Mattingly, is the difficulty he has with his preparation between assignments.
"In between starts is in between starts. You can't let that affect how your day goes," Beckett said after his first start against the Cubs since August 2005. "Physically, I'm able to pitch. I'm not worried about my hip. I felt better today, but the results are just as (poor).
"I didn't throw a bullpen in between my last two starts, because I feel like it's robbing Peter to pay Paul," he added. "I threw some flat ground, which kind of substitutes as my bullpens nowadays."
Los Angeles' NL West lead was trimmed to 2 1/2 games over San Francisco, which won 9-0 at the New York Mets.
Matt Kemp trimmed Chicago's lead to 3-2 in the sixth with his 13th homer and fifth in six games. But Cubs first baseman Anthony Rizzo helped preserve the lead in the seventh, making a leaping grab of Carl Crawford's line drive and turning it into an inning-ending double play by trapping Drew Butera off second.
The Cubs made it 6-2 in the eighth with John Baker's RBI groundout and a two-run single by pinch-hitter Starlin Castro, who was rested after starting each of the team's first 109 games. The Dodgers got a run in the eighth off Pedro Strop, but Luis Valbuena responded in the ninth against Chris Perez with his 10th homer.
Edwin Jackson (6-11) allowed two runs and seven hits, struck out six and walked none. The right-hander was 0-4 with a 7.39 ERA in his previous seven starts since beating Pittsburgh 6-3 on June 20 at Wrigley Field.
SMELL THE WOOD?
SEEMS LIKE OLD TIMES
Jackson made his big league debut with the Dodgers on Sept. 9, 2003, beating Randy Johnson and the Diamondbacks 4-1 at Arizona on his 20th birthday in a game that lasted just 2 hours and 22 minutes.
Cubs: Newly acquired left-hander Felix Doubront had a bullpen session at Dodger Stadium while continuing his rehab from a left calf strain.
Dodgers: Catcher A.J. Ellis missed his second straight game because of a sore right knee, which was struck by a foul tip off Rizzo's bat in the sixth inning of Friday's series opener. ... Butera was struck in the groin by a foul tip off the bet of Nate Schierholtz on the fourth inning and was given as much time as he needed to collect himself before going back behind the plate.
Dodgers: Their next four games are against the Angels, with right-hander Zack Greinke (12-6, 2.65 ERA) opposing Garrett Richards (11-4, 2.74 ERA) on Monday night at Chavez Ravine. The Dodgers have won only one season series from the Angels in the previous 14 years, going 4-2 in 2006.