The Londoners slipped to a 3-1 home loss on Sunday after Brede Hangeland was sent off in the first half, marking their first defeat in six Premier League games.
Petric scored Fulham's only goal in the 62nd minute and was denied a second by a great save from Sunderland goalkeeper Simon Mignolet.
Manager Martin Jol lost both Alex Kacaniklic and Ruiz to hamstring injures and Petric feels the enforced changes had an impact of the Cottagers' chances.
He Croatian said: "I think we have to learn from that game, but we had two injuries so I think that luck was not on our side.
"I think we tried very hard and we always believed we had it in us to get back in the game.
"We kept putting pressure on Sunderland and we had some good opportunities again to score but unfortunately we just couldn't find the net again."
Petric had an incident-filled game after replacing the injured Bryan Ruiz after an hour.
The Croatia international almost equalised with his first touch before finding the back of the net two minutes later after Damien Duff beat the offside trap to provide the assist.
Petric then required treatment on a wound suffered when scoring the goal and was off the field as Sunderland regained the lead.
"I got a kick on my calf when I scored," he said.
"It made a cut so I had to come off to have stitches put in and then by the time I got back on the pitch it was 2-1. It was a bit painful but I had injections so it was okay."
Jol's side travel to Stoke next weekend with Hangeland, Kacaniklic and Ruiz all missing but it is the absence of the club's captain that worries Petric most.
"Of course it will be a difficult game when we go to Stoke at the weekend," he said.
"It will be especially difficult because Brede is suspended and it is the kind of game you need your big centre-back because he's a tall player and he's very strong with his head."