Liverpool missed a big chance to secure safe passage to the Champions League last 16, important in any season, but particularly important given their upcoming schedule.
The draw means Liverpool must put out a strong side against a good Red Bull Salzburg team in a fortnight, in the midst of a run of fixtures - just the 11 games in 34 days.
It's fair to say Liverpool haven't looked fatigued to this point, but Wednesday's performance may have showed the first signs. They struggled to stretch a particularly stubborn Napoli who were happy to sit on a lead.
Their transitions were slow, the full-backs not as energetic, and the front three out of touch. It was a rare sight.
Liverpool's failure to keep a clean sheet wasn't a rare sight; they conceded for the 11th game in a row, their worst such run since 1998.
There were some worrying performances too - Joe Gomez and Mo Salah in particular - but they did show some character at least to pile forward late on. It felt like it was going to be another comeback, but instead, Klopp has a huge headache ahead.
And, as Klopp admitted after the game, the injury to Fabinho could be 'massive'.
Manchester City's safe passage to the Champions League last 16 wasn't met with the relief and celebration it warranted. The Etihad has been the home of slick football in recent years, but on Tuesday night, it was sloppy, lethargic and, above all, worrying.
Despite finishing top of their group, therefore allowing for a much-needed rest in their final group game in Zagreb, Pep Guardiola will be concerned about a hazardous defence and a frontline which struggled to create clear-cut chances.
Guardiola couldn't be accused of not taking the game seriously - he didn't look any less frustrated or concerned on the touchline as he has at times this season - but there may be some mental fatigue settling into his City side.
They let Shakhtar Donetsk in on at least four occasions, the types of mistakes and openings we were used to seeing once every two games at the Etihad.
City didn't have that intensity and speed we have all become accustomed to, and perhaps its absence looks worse because it has existed consistently for so long. But Guardiola must ensure this doesn't creep into the Premier League title race as he takes a mental break from Europe. This is a different kind of test of his skills at City.
Few turned up at the Tottenham Hotspur Stadium on Tuesday night expecting anything other than a routine home win. They got the win, but it was anything but routine.
Mourinho himself must have been flummoxed at how his team, so confident and capable at West Ham three days prior, rushed into every pass and were tentative in every challenge in a first half dominated by nerves in front of Spurs' home support.
He was constantly telling his players to calm down, and the turning point was not his first-half replacement of Eric Dier with Christian Eriksen, but rather the gift of a comeback goal handed to Tottenham on the stroke of half-time.
With some 'loving' words, in the opinion of the man himself, at the break Spurs were a different outfit after the interval, and Mourinho confidently predicted there will be no further nervous starts on home soil. But he certainly wouldn't have predicted seeing his side 2-0 down inside 15 minutes to an Olympiakos team who travel very badly, either.
If there was any hope that seasoned-campaigner Mourinho would sweep in overnight like Mary Poppins, Tuesday showed how many bumps in the road they already have to face.
Lightning is not supposed to strike twice, but for Chelsea in the Champions League, they were struck again by the thunderbolt of a dramatic European fixture. After a 4-4 draw against Ajax three weeks ago - which included a three-goal comeback and two red cards - there was more of the same at the Mestalla on Wednesday.
On the line in Spain was a place in the last 16 - the victor would have booked their spot in the knockout stages - and the immensity of that permutation showed on the pitch.
There were two quickfire first-half goals to see the scores tied at 1-1, before VAR ruled in Chelsea's favour for Christian Pulisic's goal, which was initially ruled out for offside. Add in a saved penalty and a freak equaliser from Daniel Wass, there has been more than enough heart-stopping moments for Blues fans in the Champions League this season, but they will hope there are plenty more to come.
Ultimately, it is a good result for Chelsea and they remain unbeaten in their three away games in the competition this season, which will boost their confidence should they make it into the last 16. Indeed, three teams can still qualify from Group H, with Ajax and Valencia joining Frank Lampard's side in the race for the knockout stages.
Chelsea will need to tighten things up in midfield and in an end-to-end game, getting back to defend almost cost them on a number of occasions. They were only saved by some woeful finishing from Valencia, which could well be their downfall in their showdown with Ajax in two weeks' time. At least the injury to Tammy Abraham doesn't seem serious.
But there's no doubt that all eyes will be on Group H in the final group matchday, and Chelsea will need an energetic Stamford Bridge crowd to send them into the Champions League last 16.
The draw for the last 16 of the Champions League takes place on Monday, December 16 in Nyon, with proceedings set to start at 11am UK time.