Penalties are clearly a matter of mentality and in this match Gabriel Martineli failed to carry out his role which knocked Arsenal out.
Even so, it can't be blamed either because it's also very difficult for the task of kicking.
The bright side is that they can focus more in the EPL now.
Manchester United though it looks like an easy ride for them now to the finals, they still have to be very careful with some teams that are underrated in this competition and even this Sporting boys.
I don't know if Arteta's decision not to start our regular strong first eleven players is due to team rotation or because he underestimated the boys from Sporting . For whatever reason, it looks like what cost us the game.