is there a definition that we can agree on? Otherwise... OK, so you intended different scenarios, when you said "crisis" ... I understand...
Crisis is defined as an event that could, and in football terms pretty much always does, lead to a disaster i.e. a series of unwanted results for your team form. That does not constitute a loss against RM or against Barcelona. Liverpool haven't had this, they did very well averting them by strengthening most parts of the squad such as the keeper (where they had either both of similar quality and subsequently bought Alisson) or the wings (by buying Shaqiri to supplement rotation). They did smart business. But they eventually will suffer, because there is nobody to replace Van Dijk should he go down. Same with Milner once he loses the primary reason he is here for or goes down long term. Mane is less of a problem, but still far more crucial than it was ever highlighted.
[MENTION=15262]serghei[/MENTION]
Why are you even bringing up the CL? This doesn't devolve just down to a big player being out in a big match. And it's actually more important for regular league fixtures that happen every week for which players are less likely to recover for quickly and hence injury of a key player can, and often does, cause big issues to team form. Ronaldo was never as key to Real Madrid winning league games, many times he was out we coped just fine. It was more often players like Alonso, Modrić, Casemiro, Marcelo or James that caused us to lose shape for stretches of league fixtures. Same with Barcelona having big issues away without Messi (until this season where it's just become a routine) or back in the day Alves, Abidal or Busquets.