Brown cut the margin to 3-2 at 18:33, beating Petersen to the left on a wrist shot from the right circle.
Ottawa could not capitalize on a five-minute power play that began at 13:51 of the third period. Brendan Lemieux received a match penalty for his part in a confrontation with Brady Tkachuk.
"It's not a good situation to put the team in, and not only for tonight but moving forward," McLellan said. "Brendan Lemieux has become a really valuable player to our team. I think he knows that. We all believe it, and we need him in the lineup. We need him playing every night. We need him playing every minute that he gets. Offensively, defensively, physically, emotionally. And if we happen to lose him for any period of time, we'll miss him. And he has to understand how important he's become."
Grundstrom came out of the penalty box to score into an empty net at 19:26 for the 4-2 final.
"It's just details," Paul said. "A couple mistakes this road trip ended up in the back of our net, and then the momentum changes. I think we just got to be detailed and hard and stick together."
Viktor Arvidsson put Los Angeles ahead 1-0 at 4:59 of the first period when he was in the slot to tip Kopitar's pass on the power play. It was Arvidsson's third power-play goal, with Kopitar picking up his eighth point (three goals, five assists) with the man-advantage.
Ottawa tied it 1-1 at 19:41 when Del Zotto's shot off a face-off win by Josh Norris deflected off Alex Iafallo's stick and over Petersen.
Del Zotto, who had been scratched the previous two games, has scored six points (two goals, four assists) in 10 games this season.
NOTES: It was the 250th multipoint game of Kopitar's NHL career. Since coming into the NHL in 2006-07, Kopitar ranks sixth in multipoint games, trailing Sidney Crosby (366), Alex Ovechkin (354), Evgeni Malkin (299), Patrick Kane (293) and Nicklas Backstrom (258). … Kings rookie defenseman Sean Durzi got his third NHL point in two games (one goal, two assists), all of which have come on the power play. … The Senators claimed center Adam Gaudette off waivers Saturday. Gaudette scored two points (one goal, one assist) in eight games for the Chicago Blackhawks this season. … Los Angeles swept the two-game season series; it won 2-0 at Canadian Tire Centre in Ottawa on Nov. 11. … The Senators played the second of back-to-back games. They lost at the Anaheim Ducks 4-0 on Friday.