The Cottagers striker believes his side were unfortunate to leave the game empty-handed on Sunday following Brede Hangeland's dismissal and an injury to playmaker Bryan Ruiz
The 31-year-old equalised on 62 minutes after coming off the bench following Steven Fletcher's opener, but Martin Jol's side could not prevent Carlos Cuellas and Stephane Sessegnon securing three points for the Black Cats.
And the Croatian striker blamed the loss on bad luck after captain Brede Hangeland was shown a straight red for a first-half challenge on Lee Cattermole, with Bryan Ruiz and Alexander Kacaniklic departing with injuries.
"It was a hard game for us because it started very badly with the red card for Brede," Petric told the club's official website.
"Then we were 1-0 down so it was good to come back and equalise, although they scored two minutes after my goal which was a pity.
"I think we have to learn from that game, but we had two injuries so I think that luck was not on our side."
Petric was temporarily withdrawn after being injured himself, but played through the pain to ensure Fulham didn't finish the game with nine men.
"I got a kick on my calf when I scored," Petric added. "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.
"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."
Despite the defeat, the Croatian feels the Cottagers can trouble Stoke in their next game, and believes Philippe Senderos can fill in for the suspended captain Hangeland.
He continued: "Of course it will be a difficult game when we go to Stoke at the weekend. 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.
"But Philippe [Senderos] played his first game of the season on Sunday and I think he did well. He's good in the air as well, so I think we have no reason to be scared going into the match. We have a lot of quality in the team to play a good game there."