It was a horror show from Celtic on Tuesday night as defensive ineptitude coupled with a lack of composure in front of goal gave them a mountain to climb in Glasgow next Wednesday night.
The Hoops trail Shakhter by 2-0 and, while Neil Lennon's side looked shell-shocked as they trooped off the park, Petta is optimistic they will regroup and get the required result next week.
Citing the wave of support Celtic will be playing in front of and the pressure they are under as catalysts for hitting the ground running at Celtic Park, the Dutchman expects a display of much greater composure next week.
"I think everyone was gutted at the performance," said Petta. "But I still think Celtic can get through.
"You would be crazy to write them off just now. It is just half-time and, while they will be disappointed at the way they played - and no-one will know that more than Neil - they still have it within their power to get it back. Celtic Park on a European night is a little bit crazy.
"The supporters will be right behind the team, Neil will have them fired up for it and, if they can get a few of their injured players back and looking a bit sharper, then they will certainly be capable of scoring goals.
"If they come out and get an early goal it will lift the roof off the stadium. But while they need to keep their head in front of goal, they really must do better defensively.
"It is the poorest Celtic have looked at the back for a long time. That will be uppermost in the thoughts of the manager because they simply cannot afford to lose another goal.
"They need to be strong and they need to believe - but they also need to get their heads down and get some work done this week if they want to overturn the scoreline."
Not helped by the manner in which Virgil van Dijk and Steven Mouyokolo played, Fraser Forster, too, looked jittery between the sticks, failing to come off his line or command his box.
The goalkeeper, who harbours ambitions of making it into Roy Hodgson's England squad, performed to nowhere near the level he is capable, and that anxiety at the back seemed to spread throughout the spine of a Celtic side which lost their composure.
There were chances, though, that would have changed the entire complexion of the tie - Kris Commons cannoned one attempt off the crossbar, Charlie Mulgrew had a couple of opportunities, the two centre-backs could have set the tone for a comfortable evening by heading into the net early doors, while James Forrest will still be replaying the header that he sent directly into the keeper's arms when he had both time and space in abundance.
Yet, for Petta, Celtic have to be more mindful of how they utilise the space they will get at Celtic Park - but he has also called for a better delivery from the flanks.
One of Petta's most outstanding displays for Celtic came in the Amsterdam Arena in August, 2001, when Martin O'Neill's side clocked up a 3-1 win over Ajax en route to their first-ever appearance in the group stage of the Champions League.
And he believes that mental strength will be as important as any other attribute when Celtic take to the field.
"I felt the crossing could have been much better on Tuesday night," he said.
"Time after time the ball into the box wasn't good enough, or wingers were in a one-on-one situation and you were crying out for them to go past the guy, to drive on and they didn't.
"This game next week is for a place in the Champions League. As a player, it is the biggest competition you can play in during your club career.
"You need to be bold and you need to be brave on the ball and that is where I felt Celtic didn't show enough in Kazakhstan.
"It will be difficult next week because Shakhter will sit in.
"They will defend for their lives because they are 90 minutes away from going through to the group stage of the the tournament for the first time.
"I have played in games like that and it is hard trying to find space, to break teams down.
"That is where you need to use the width and it is also where you need your wingers to show a bit of creativity and courage."