We're worried because this happened early in the season and we don't know if we do have a problem in defense or it was just an accident. Normally, although not very often, these things happen to every team. Last year we were having fun with United, just to concede two late goals and then Martial missed 1 on 1 for a third goal. Or remember those CL encounters from the previous season, when teams like Atletico Madrid, Barcelona and Ajax were losing 2 and 3 goals advantages, even if to some of them it happened in the 2nd leg, not on the same match.
If this match happened in round 24, it would have provoked a different reaction.