Juventus was stunned by Lech Poznan's stoppage-time equalizer, defending champion Atletico Madrid lost at Aris, and Liverpool comfortably beat Steaua Bucharest to open the Europa League group stage on Thursday.
American forward Jozy Altidore played his first full match for Villarreal this season as the Yellow Submarine lost 2-0 at Dynamo Zagreb.
Juventus, a two-time European champion, trailed by two goals but went ahead as Giorgio Chiellini scored twice and Alessandro Del Piero once. Lech Poznan's Artjoms Rudnevs completed a hat trick to give the Polish team a 3-3 tie.
Manchester City enjoyed a smoother ride in the other Group A match, with goals from David Silva and Jo earning a 2-0 victory at Red Bull Salzburg.
Five-time European champion Liverpool only came to life in the second half, with David Ngog scoring twice in the 4-1 victory over Steaua Bucharest in Group K. Joe Cole and Lucas Leiva also had goals for the Reds.
Four months after ending its 14-year wait for a trophy, Atletico endured a miserable start to its title defense in a 1-0 loss to Aris, which got a second-half goal from Javito.
Altidore rejoined Villarreal this season after 1½ seasons on loan. When he appeared in Europa League qualifiers last month against Dnepr Mogilev, he was making his first appearances for the Yellow Submarine since Dec. 14, 2008, when he was a 57th-minute substitute at Sevilla.
Altidore joined Villarreal from the New York Red Bulls in June 2008. After only two starts and four substitute appearances during the season's first half, he was loaned to Xerez in the second division on Jan. 30 and didn't get into another game that season.
He spent last season on loan to Hull and scored just twice, against Southend on Aug. 25 in the League Cup and against Manchester City in a Premier League match on Feb. 6.
With the U.S. national team, Altidore has nine goals in 30 appearances.