Jan Vennegoor of Hesselink believes Celtic need good fortune and bad weather to beat Barcelona in Glasgow on Wednesday night.
The former striker put Celtic in front in the round-of-16 first-leg clash with the Catalans in February 2008, but Barcelona responded to win 3-2 and claimed a 1-0 home success to advance.
The Dutchman said: "You have to be realistic. It's going to be difficult, but at lot can happen at Celtic Park. Hopefully it will happen on Wednesday. They have a chance. They play Barcelona and everyone knows their qualities and their players."
He added: "You hope for Celtic that (Barca) have a really bad day and that you have a really good day, that it rains hard, that they are cold, that the boys in Scotland are used to it and that they can get a result. It's going to be hard. You need a little bit of luck."
Barcelona lead Champions League Group G with a 100% record from three games, including a last-gasp home win against Celtic.
Neil Lennon's men are second after claiming a first away win in the competition at Spartak Moscow and a draw with Benfica.
Vennegoor of Hesselink is uncertain if his former side will reach the knockout stages at the halfway stage of a competitive pool.
He said: "It's a difficult group, with Barcelona on top. The two other opponents invested in a lot of players. If you're a little bit lucky you can progress, but it's going to be difficult. When we got to the second round twice in two years we had luck on our side too."
Vennegoor of Hesselink retired in May after his second spell at PSV Eindhoven, but many of his finest moments were with Celtic.
Of the powerful header against Barca, he said: "I saw it yesterday for the first time in a long time. Good memories. Shame we didn't win the game."
- Sports & Recreation
- Jan Vennegoor of Hesselink