Getting swept by the division rival Giants simply confirmed the Los Angeles manager's fears.
A day after giving up the winning run in the ninth inning, the Dodgers let another game slip away late when Justin Maxwell singled home the winning run with one out in the 10th inning and San Francisco beat Los Angeles 3-2 on Thursday.
Six relievers combined to give up five hits and two runs following a solid outing from starter Mike Bolsinger in his Dodgers' debut.
"We're asking a lot of different guys to pitch late in the game," Mattingly said. "That's what we knew we were in for when we left spring training without Kenley. Would I like to have Kenley back? Yeah, no doubt about that, and we're going to get there. But our guys, I felt like, have done a nice job."
Jansen, who recorded a career-best 44 saves in 2014, only recently began throwing off the mound and isn't expected back until mid-May at the earliest.
Until then, the Dodgers will continue to work with a by-committee approach for a closer.
Santiago Casilla (2-0) pitched the 10th for the win as the Giants swept the Dodgers at home for the first time since May 2013. The defending World Series champions have won four of five following an eight-game losing streak.
The Dodgers had brought a seven-game winning streak into AT&T Park this week. They dropped the opener 6-2 before losing consecutive one-run games.
"We've got a solid bullpen," catcher Yasmani Grandal said. "These things aren't going to happen too many times."
Pagan, Maxwell and Joe Panik all had two hits for the Giants.
Alex Guerrero homered and singled and is batting .474 (9 for 19) with four homers and 12 RBIs for the Dodgers. Adrian Gonzalez also homered.
Dodgers second baseman Howie Kendrick made a diving catch in the eighth that preserved the slim lead. The Giants had the bases loaded with two outs when Maxwell hit a line drive up the middle that Kendrick snared just off the ground.
Bolsinger, who did not allow a run in 11 minor league innings, took a shutout into the sixth. He finished allowing one run and five hits. He walked two and struck out five.
Bolsinger, the reigning PCL Player of the Week, was recalled from Triple-A Oklahoma City to make the start.
Giants starter Ryan Vogelsong gave up two runs and three hits in six innings. He walked one and struck out five.
In the sixth, Bolsinger gave up a one-out walk to Panik and a single to Pagan. After Posey struck out, Belt singled home a run.
Dodgers: C A.J. Ellis, pulled from Wednesday's loss with a bruised right hand, did not play. X-rays were negative and he's expected to be ready by Friday.
Giants: OF Hunter Pence (fractured left forearm) is swinging off a tee with a wood bat. He had been using a Wiffle bat. ... RHP Erik Cordier (right forearm strain) has made three rehab appearances for Single-A San Jose, totaling three scoreless innings with seven strikeouts. INF Travis Ishikawa (lower back strain) had a setback and will be rested.
Dodgers: RHP Zack Greinke (2-0, 1.83) takes the mound against the Padres in San Diego on Friday. He's 2-0 in with a 1.74 ERA in five career starts at Petco Park.
Giants: RHP Chris Heston (2-1, 0.87) gets the start against the Rockies on Friday in Denver. Heston is the first Giants rookie to start a season with outings of at last six innings and one or fewer earned runs since 1914.
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