United travel to Turin to face the Italian champions in Champions League play on Wednesday, having already lost 1-0 to Juve at Old Trafford two weeks ago.
Juventus have won seven consecutive Serie A titles, and have started the 2018-19 season strongly by winning 13 of their 14 games so far.
Darmian, who played for Juve's city rival Torino between 2011 and 2015, is expecting a huge challenge for United at the Allianz Stadium.
“I think Juventus are a very big club, one of the biggest clubs in the world,“ the 28-year-old told MUTV.
"For sure, they are the biggest club in Italy and they have won the last seven Serie A (titles) so everyone wants to beat them. They are a very good team and it will be a difficult game.
“They have good organisation and are a compact team so it’s difficult to score against them, so we have to be focused and try to score and win the game.“
Juventus bolstered their squad in a major way over the summer, landing former United star Cristiano Ronaldo in a big-money transfer from Real Madrid.
The Portuguese star has adjusted well to life in Italy so far, scoring seven goals and adding six assists in 13 games with the Old Lady.
Although Darmian hailed Ronaldo as the biggest coup of the transfer season, the defender knows that Juve are anything but a one-man team.
“Cristiano was the biggest signing of the summer transfer window,” Darmian said.
“We all know the quality of Cristiano, but he’s not the only one we have to look out for because they have a lot of good players, not only up front but also in midfield and defence.”