The quality of the two players has a balance, which means that the match will be fierce and difficult to predict, it's just that Barcelona has the advantage of playing at home in the first round, not only Lewandonski Barcelona also has the strength of young players who can trouble Manchester United's defense, but the biggest role what can distinguish only the strategy that will be implemented by the coach when the two will meet at Camp Nou, I think the strategy implemented by the coach is the key to victory.
Manchester United also have Casimero who knows Barcelona's playing style by heart and they also have Marcus Rashford whose performance is getting sharper up front, Erik Ten Hak can take advantage of the strengths they currently have, at least being able to hold Barcelona to a draw, so that Manchester United can play openly and take chance to beat Barcelona at Old Trafford in the second leg.
This is a bit tense, yes I think this match will really be the best match for both teams. Barcelona and Manchester United were both coached by new managers, but maybe Xavi understands better how to make his team really able to win match to maintain his good record against Manchester United.
Even though Manchester United had Casemiro, that never guaranteed to beat Barcelona. Believe me, even Real Madrid couldn't stop Barcelona in the final, but that doesn't mean all Barcelona's plans will go perfectly. Both teams have to avoid lot of mistakes and both exploit the opponent's weaknesses, but it seems that Barcelona is the team that is expected to be able to control the game well.