Bayern Munich s German midfielder Leroy Sane celebrates after scoring a goal during the UEFA Champions League Group C football match between Inter Milan and Bayern Munich in Milan, on September 7, 2022 (AFP)
Leroy Sane fired Bayern ahead in the 25th minute and Inter defender Danilo D'Ambrosio netted an own goal in the 66th. It was the 19th straight time the German team has won its opening match in the competition.
Barcelona beat Viktoria Plzen 5-1 in the other Group C match.
It was further disappointment for Inter, which has already lost two matches in Serie A, including the derby against AC Milan over the weekend.
Indeed, Inter coach Simone Inzaghi had made several changes from that match, notably dropping Samir Handanovic and handing a debut to Onana, who had not played a competitive match since April at Ajax. Star forward Romelu Lukaku was still injured.
Onana was kept busy from the start as he had to make two saves from Joshua Kimmich within three minutes. He also had to get down smartly to stop an effort from Thomas Muller, while Kingsley Coman fired narrowly over the bar.
Inter only had a couple of sights of goal in the first half, with Lautaro Martinez flashing a shot across the face of goal early on and a weak effort from D'Ambrosio that was straight at Manuel Neuer.
Onana pulled off a great save to palm Muller's shot over the bar but could do little when Sane ran between two Inter defenders and onto a ball over the top from Kimmich, rounded the goalkeeper and deposited into the empty net.
It was another defensive lapse from Inter, which has already conceded eight goals in five league matches this season.
Bayern was in complete control and indeed had a triple chance to double its tally as first Onana parried an Alphonso Davies effort, then D'Ambrosio blocked Muller's follow-up shot and Marcel Sabitzer fired the rebound narrowly wide of the left post.
Inter appeared to have a different approach after the break and created chances but D'Ambrosio and Edin Dzeko failed to make the most of early opportunities.
A comedy of errors almost gifted Bayern its second as Alessandro Bastoni attempted to head Kimmich's shot back to his goalkeeper but perhaps did so too powerfully as Onana had to backtrack rapidly to claw it out from under the crossbar and off the post.
Moments after risking it, the Nerazzurri did score an own goal. Coman and Sane combined beautifully on the edge of the area and the latter's cross-shot was turned into his own net by D'Ambrosio.
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