Barcelona's stunning Champions League loss to Roma serves as a warning to Arsenal as they prepare to defend the same advantage against CSKA Moscow in the Europa League.
Ernesto Valverde's men carried a 4-1 first-leg lead to Rome but were sensationally denied a place in the semi-finals eight minutes from time when Kostas Manolas sealed a remarkable 3-0 win for the Serie A side.
Similarly, CSKA are three down but with an away goal to their name ahead of hosting Arsenal in the Russian capital on Thursday, with a place in the final four of the Europa League up for grabs.