The Dutch champions lost a number of key players in the summer and captain Siem de Jong was, at 24, the oldest player in their ranks during their Eredivisie clash with FC Twente on Saturday.
Former Holland striker Denis Bergkamp, who is now on the coaching staff at the Amsterdam club, has questioned whether there are enough hard men in the team, although Danish holding midfielder Christian Poulsen is set to return at Parkhead.
And Dutchman Petta feels physicality will be the key weapon for Celtic, who have lost Kris Commons to a hamstring problem.
Celtic sit bottom of the group following defeats by AC Milan and Barcelona in their opening two games, one point behind their opponents.
Petta, who scored in a 3-1 away victory over Ajax that sent Celtic into the group stages in 2001, said: "We had quite a good strong team and had quality all over the place. We could play from the back, from the wings, we could play to the strikers, and we had good midfield players who could make good runs and finish. We had a variety of players.
"If I look at the Celtic team now, there are a lot of players who are very similar. I think they lack a bit of creativity in my opinion."
Petta would not write off Celtic, though, in the wake of Bergkamp's comments.
"Denis knows the values of his team, so I would not argue with that," Petta said.
"They are a very young team and they might not be used to the type of football Celtic can play.
"Celtic have to play to their physical strengths."
But Petta, who started his career at Feyenoord, added: "Ajax always create chances. They will have a different style from Celtic, they will keep moving the ball about."
Petta's goal in Amsterdam was one of the highlights of his Celtic career and he urged the players who come in for the likes of Commons and the suspended Scott Brown to seize their chance.
"In my opinion it's a must-win game," Petta said. "They can't just go for it from the get-go but they have to set their intentions out early and put Ajax under pressure. Celtic Park is a daunting place on a European night.
"Playing against Ajax in the Champions League, you can't ask for a better opportunity for those players to come in and do well for the club. I would certainly embrace that.
"They just have to go out there and do what they do best and play freely. There is always pressure but it's a good pressure to have.
"There is nowhere else you would rather be."
:: Petta was speaking on behalf of Celtic's main sponsor, Magners, at the Wellpark Brewery in Glasgow where fans can enjoy a free mini-tour of the facilities from 2.30-6.30pm on Tuesday before the game.