Arsenal knocked Porto out of the Champions League in a fiercely contested two-legged tie, but they were far from their usual selves. Mikel Arteta’s side have been scoring freely in the Premier League, but managed just one goal across two games and extra time against Porto, and needed penalties to reach the quarter-finals. Arsenal found themselves unable to carve Porto’s stubborn defence open, and were also frustrated by their ability to run down the clock and waste time. Despite some of Arsenal’s attackers struggling, Arteta did not make a single substitution until the 83rd minute, when Gabrie…