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Full defensive is very good tactical for FC Porto on second leg match against Arsenal, get capital winning over one goal and draw result on second leg match more enough for FC Porto qualifying to quarter final although Arsenal's advantage with playing in their home. I think will make frustrated with the opponent team when FC Porto make full defensive strategies on second leg match.
I think FC Porto can take advantage with counter attack with Arsenal will play more offensive make space with their defense position, be careful for Mikel Arteta if Porto will play more defensive and Arsenal payer get frustrated how difficult scoring goals make their defense players will leave their position.
Regardless of strategy, Porto only needs to end the match with a draw or win to qualify for the quarter-finals. However, it is unlikely that this plan will easily come true because Arsenal will try to maximize their home advantage to win the match. It is this away game that is the real challenge for Porto, even if they have 1 goal in hand, it is not much.
Arteta is likely to change his playing strategy after learning something in the first leg against Porto. He knows Porto is strong in defense, so Arteta will adjust his strategy to damage Porto's defensive dominance. Maybe it was too risky for Porto to use the bus parking strategy, it was not a very wise choice when they had a chance to play a little offensively.