Alessandro Del Piero has said Cristiano Ronaldo deserves huge credit for Real Madrid's three successive Champions League titles and tipped Ronaldo's new club Juventus as potential winners this season.
Del Piero hailed Zinedine Zidane's achievements as Madrid coach but said Ronaldo had been central to their run of trophies.
"Even if the coach's merit in winning titles is greater than the players, I don't think there is any coach more important than CR7 and I say that even though I think Zizou [Zidane] did an extraordinary job -- above all expectations," he told La Gazzetta dello Sport.
"He's an extraordinary champion and has the mentality that only the greatest sportspeople have, and he will know how to make it contagious."
Del Piero -- part of the last Juve side to lift the Champions League trophy in 1996 -- said he believed they must be considered among the favourites this season.
"Real Madrid are on the first row, obviously since they've won three in a row, even if they've lost Ronaldo," he said.
"On the second row, there are Juve, [Manchester] City and PSG, then on the third row there's Barcelona, Bayern [Munich] and Liverpool.
"I hope to see [all four Italian clubs involved in the round-of-16] and it is possible but, with a bit more realism, I would have to say I'd be happy if three of them went through and another went into the Europa League. Napoli have the toughest group."
Del Piero, however, backed Napoli coach Carlo Ancelotti to improve the club's fortunes in Europe.
"Ancelotti is a No. 1 in Europe," he said. "He brings a lot in terms of mentality and knowledge, but it would be unfair to blame [former coach Maurizio] Sarri for their difference in performance between the league and Europe.
"It's part of a pathway and now it's Carlo who has to manage this next stage of development, aiming to get the best he can out of his entire squad to ensure they aren't short of energy at the key moments in the Champions League."
Del Piero said Roma, who lost to Liverpool in last season's semifinals, can "be a surprise again," adding that they had "the right mentality to become something of an Italian Liverpool -- a side who exceed themselves in Europe."
Juventus travel to Valencia in their Champions League opener, while Inter host Tottenham Hotspur, Roma visit Real Madrid and Napoli are at Red Star Belgrade.