The Bianconeri have drawn both of their matches against Copenhagen and Galatasaray in the group stages of the competition so far.
But Vialli feels Antonio Conte's men can be successful in the upcoming fixtures with los Blancos, and turn the momentum in their favour.
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"If they were to get a result in Spain, it would change the course of the season, going beyond all expectations.
"The one with the Real could be the turning point of the race, but in the end I think for Juve the most important challenge in the group may be the one at Galatasaray.
"I think that they will do well in this Turkish adventure, Juve will have to be careful, however.
"At Juventus nothing is missing to make an impact in the Champions League."
Juventus have won back-to-back titles in the Serie A in 2011-12 and 2012-13, but the former attacker is convinced that the league will be more balanced this term.
"For Juventus, this year is much more difficult than the last two seasons.
"On one side there are two to three teams that have strengthened a lot, thus becoming competitive for the championship.
"On the other hand opponents now know the strengths and weaknesses of Conte's team and have begun to take countermeasures."
Table-toppers Roma will meet second-placed Napoli on Friday, but Vialli does not think the match will be an early challenge for the league title.
"At this time the Roma are very comfortable, but Napoli work great as a collective. It will be a good game, but it is too early to consider it a challenge for the Scudetto already."