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Manchester City is Arsenal's main enemy in the Premier League
Meanwhile in the Europa League, several teams such as Real Sociedad, Freiburg, Betis will be at the forefront of prevent Arsenal from winning the Europa League title this season. So far I'm still pretty sure Arsenal are the strongest candidate in the Europa League
I think Arsenal can win both the Europa and Premier League all the need is more more good games and consistency. They maintained a good performance and stat in the premier league as well as Europa in the early games of the season. The remain the favourite to win the Europa but for the PL the will have to maintain the point difference the have over Manchester City because Pep is cooking up some new tactics to test them once the season resumes.
if MU shows good performance in the European league then I think it will be a tough thing for Arsenal to be able to smoothly get the European league trophy this season. there are many favorite teams in the European league, apart from the arsenal, there are Barca, MU, and Juve who are favorites, arsenal's mission is really tough.