Croatian hitman Petric has a hamstring problem which he picked up when replacing Dimitar Berbatov half an hour into last week's 1-0 defeat at Everton and will not be available.
However boss Martin Jol is boosted by the news that Berbatov has recovered from the calf problem which forced him off at Goodison Park and will be back in the side against the already relegated Royals.
Sidwell is still suspended following his red card against Arsenal, but defender Sascha Riether is fit for selection following an ankle injury.
Midfielders Mahamadou Diarra, Ashkan Dejagah and Simon Davies remain out with respective knee, ankle and hip injuries.
Meanwhile Alexander Kacaniklic has urged his team-mates to get back to winning ways and finish the season on a high note.
He told the club's website: "We need a win on Saturday. It's a home game in front of our own fans so it would be nice to get back to winning ways at the Cottage. We haven't won for a while now so at the weekend we won't be looking for anything other than three points.
"There are three games left and we want to win every one of them. That should always be the mentality - to go into every match and want to win.
"That's what we need to do, first of all on Saturday, and then again when we face Liverpool and Swansea City."
On Reading, he added: "In their current situation they have nothing to worry about. If they lose, they lose, but if they win, that'll be good for them. It'll be a dangerous game."