PITTSBURGH — The Washington Capitals went into PPG Paints Arena on Sunday afternoon healthy, rested and eager to get back on track after an unexpected week-long break.

And with a full cast of skaters available for the first time in nearly four weeks, the Capitals came out with energy and set the pace early against the Pittsburgh Penguins. But the chance to break their losing streak slipped away amid multiple lulls and shaky goaltending.

The Capitals fell to the Penguins, 6-3, and have lost four straight games in regulation after a 6-0-3 start to the season.

“[We’re] concerned about every game, but certainly I am not concerned about our team,” defenseman John Carlson said. “I think we got pieces in there that know what to do. . . . We are losing, and no one is accepting that, but I think our game is getting better. Feels like we are going longer stretches of going the right way.”

Washington will get another chance to have things go the right way against the Penguins (7-5-1) when the rivals meet again Tuesday night at PPG Paints Arena.

If Vitek Vanecek is in the net for that game, it would be his 11th straight start. After allowing four goals on 36 shots Sunday, he has given up three or more goals in nine of his past 10 starts. The Capitals have allowed three or more goals in six straight games and in all but two games this season. They have allowed 22 goals in the four-game skid.

Washington was down 4-3 entering the third period after Nicklas Backstrom’s late second-period score gave the Capitals a glimmer of hope. But it could never find the equalizer, and Jake Guentzel’s second-period goal held up as the game-winner. Zach Aston-Reese and Sidney Crosby scored empty-netters for the final margin.

“We knew coming in here with regard to the way Pittsburgh plays, they’re low with chances. They’re low with opportunity,” Capitals Coach Peter Laviolette said. “You’ve got to fight for your space. The push probably could have been better in the third to do that. I think the mind-set was right to go out there and to try and push.”

The Capitals played for the first time in seven days after three games were postponed because of coronavirus issues with their opponents. Washington's game Tuesday against the Philadelphia Flyers was postponed hours before scheduled puck drop. The team's two-game road set against the Buffalo Sabres also was pushed back because of covid-19 guidelines.

The break gave Washington some time to rest and recover.

“We felt like we had sharp practices,” winger Jakub Vrana said. “We did everything right at the training. We were ready. Just today it wasn’t there.”

Washington had its full roster available Sunday for the first time since Jan. 19, the last time the Capitals played Pittsburgh. Four players — Alex Ovechkin, Evgeny Kuznetsov, Dmitry Orlov and Ilya Samsonov — were added to the NHL’s covid-19 protocol list Jan. 20 after Samsonov tested positive for the coronavirus. Then the Capitals dealt with injury after injury.

Nearly four weeks later, everyone is back on the ice in some capacity. Ovechkin and Orlov were cleared Jan. 30. Kuznetsov made his return to the lineup Sunday after he missed eight games on the covid-19 list.

Samsonov was removed from the NHL’s covid-19 list with Kuznetsov last Monday, but he was sent down Sunday to the team’s American Hockey League affiliate, the Hershey Bears.

Because he was away from the ice for so long after testing positive for the virus, the team wanted Samsonov to play in the AHL before returning. There is no timeline for the young Russian netminder, who was in the net Sunday against Lehigh Valley and allowed five goals on 29 shots in a 5-4 overtime loss.

Other players who returned included Justin Schultz, who missed four games after he took a puck to the face Jan. 28 against the New York Islanders. T.J. Oshie was also back in the lineup after he missed a game with an upper-body injury. Vrana, who was briefly on the covid-19 protocol list, also returned after missing one game.

Vrana confirmed after the game that he was put on the list after he “didn’t feel right.” He reported that to the team and decided he didn’t want to risk his symptoms developing into something more.

“I [could have] potentially been positive and get somebody else sick, and then the whole team miss games,” Vrana said. “I just wasn’t feeling right. I think that was the right thing to do, and now I am back on track.”

Washington avoided another major injury Sunday after Crosby appeared to have an ugly collision with Nic Dowd near the boards. Crosby kicked out Dowd’s left leg, and the Capitals’ fourth-line center was slow to get up and limped down the tunnel to the locker room. While the injury looked significant, Dowd returned to the ice about two minutes later.

Despite the rest and the returns, the Capitals couldn’t muster enough offense for a comeback.

The matinee kicked off with Kuznetsov scoring his second goal in five games this season on a power play 5:18 into the first period. The puck redirected off John Marino’s stick and past unassuming goaltender Tristan Jarry.

Bryan Rust tied the score with 9:31 left in the first period. The play turned in Pittsburgh’s favor after an ill-timed turnover by Orlov.

Pittsburgh scored again 32 seconds later with Brandon Tanev’s rebound goal, shortly after a faceoff win by Teddy Blueger.

“[We] will continue to work at it to be better defensively, but from the beginning of the year when we were giving up I think more numbers, more quality, more zone time, at the end of the day, the chances, they’ve got to continue to be worked on and be eliminated,” Laviolette said.

Vrana tied the game at 2 only 3:26 into the second period with his backhand putback in front. It was Vrana’s first goal in seven games and his fourth of the season. He has three points (one goal and two assists) in his past three games.

Rust scored his second goal of the game with 13:16 left in the second period on the power play to give the Penguins a 3-2 lead. Guentzel’s goal at 9:16 pushed the Pittsburgh lead to two before Backstrom scored with 2:11 left in the second period. The Capitals couldn’t find the tying goal in the third to at least force overtime.

Backstrom, who has 250 career goals, has seven goals and 11 assists in 13 games this season, both of which lead the Capitals.