#1 Quality of other contenders

Jurgen Klopp, Pep Guardiola, Jupp Heynckes, Massimiliano Allegri.
The caliber of coaches and the teams they lead into the last eight this year is remarkable. Only one out of these four coaches had been in the tournament last year at the quarter-final stage, and Leicester City had sneaked in at the expense of some of the bigger names.
After Chelsea’s win in 2012, each of the champions since – Bayern (2013), Real (2014), Barca (2015), Real (2016), Real (2017) – have had an air of invincibility through their UEFA Champions League campaign; this year matters are much less straightforward.
They may have gotten away with a decent quarterfinal draw against Roma, but prospectively waiting for them in the semifinals could be teams that lead their domestic leagues by 16 points (Manchester City) or 17 points (Bayern).
Won’t be easy getting past that.