Roma scored three goals in eight minutes as the Serie A side twice came from behind to stun Malcom and Barcelona 4-2 at the International Champions Cup.
All eyes were on Malcom after he opted to join LaLiga champion Barca, which dramatically hijacked Roma's €40million deal to sign the highly-rated Brazilian from Bordeaux last week.
Malcom – who was preparing to fly out to the Italian capital after Roma reached an agreement with Bordeaux before Barca stepped in at the last minute – was in the thick of the action on Tuesday, scoring the easiest of goals to put the Spanish giant 2-1 up.
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Unmarked at the back post, the 21-year-old's tap-in four minutes into the second half restored Barca's lead after Stephan El Shaarawy cancelled out Rafinha's sublime opener before the break.
But, Roma completed a late comeback – Alessandro Florenzi equalising in the 78th minute before quickfire goals from Bryan Cristante and Diego Perotti turned the match on its head at AT&T Stadium.
After sealing Barca's penalty shootout win over Tottenham last week, Malcom made his first start, while Arthur – who enjoyed a goalscoring debut against Spurs – was also in the XI, along with Nelsen Semedo and Clement Lenglet.
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There was a Roma debut for Sweden goalkeeper Robin Olsen, who was signed to replace Alisson after the Brazilian's record-breaking move to Liverpool. Daniele De Rossi, Edin Dzeko, Kostas Manolas, Kevin Strootman, Florenzi, Lorenzo Pellegrini and recruit Justin Kluivert also featured in a strong lineup.
Roma was routed 4-1 by Tottenham last week and last season's Champions League semifinalist looked in disarray against a team it sensationally eliminated in the quarterfinals.
Barca almost hit the front inside a minute, but Munir El Haddadi's scuffed shot looped over Olsen's hand and just wide of the post, however, the Spaniards were not to be denied five minutes later.
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Roma was helpless to deny Barca, who produced a jaw-dropping move that was capped by Rafinha after the Brazilian combined with Munir before receiving the ball back and volleying past Olsen.
Barca nearly doubled the lead when Malcom charged through Roma's defence in the ninth minute, but Olsen was on hand to prevent his team from falling further behind.
For all of Barca's attacking brilliance, it was exposed defensively after El Shaarawy equalised against the run of play 10 minutes before half-time – the Italian forward the beneficiary following Kluivert's darting run down the wing towards the penalty area.
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Barcelona retook the lead within four minutes of the second-half restart, however, thanks to Malcom, who tapped into an empty net from Ballou Tabla's centred pass across the six-yard box.
But, Barca was pegged back again with 12 minutes remaining when Florenzi scuffed the ball into the back of the net, despite Perotti taking too long to unleash a strike on goal.
Cristante, then, nipped in front of his opponent before beating Jasper Cillessen with a low shot in the 83rd minute, while Perotti's spot-kick, three minutes later, sealed the victory after Patrik Schick won a penalty.