If not now, Arsenal would almost certainly have to go through Atletico in the final to win the cup, if Costa is out with a hamstring, now is the time to play them.
I originally thought the match was next week and that Costa's hammie might keep him out. But he's now got 2 weeks. Don't know what his recovery history is, but it would be just our luck for him to be super motivated to come back to continue tormenting us. At least he won't be able to target our buddy Gabriel any more. More recently I recall Holding handling him well in some fixture, even laughing at him to his face. So, if Diego plays, then perhaps Kos + Holding would be a good shout.
One positive thing that's been bouncing around my head for a while is this: For this season and the last (it's hard for me to conjure details much further back than that) we've shown a great ability to come back from deficits... To not let our heads go down, when we go a goal or two down. To keep up our effort and our tempo, right through to the end of the match (apparently our match fitness is relatively good compared to some of our opponents). Doggedness, confidence, mental strength, belief, yada yada. Last night was just one example. But there've been many more in the last few years (I don't have time to look them up). And I do recall that this ability has not always been the case going further back in time. Of course, one could cynically say that we shouldn't be digging our own holes so much, which then force these dramatic comebacks. OK, but... I think credit is due to both the players and the coaching staff for this strength. And perhaps it dovetails with the team unity which has also seems to be one of our hallmarks.