Liverpool ground past AC Milan, Manchester United slipped to a damaging defeat in Switzerland, City hit Leipzig for six and Chelsea flexed new muscle.
Here, the PA news agency looks at the big lessons from the week's Champions League action.
Henderson proves his point
Jordan Henderson put his hand to his ear and wheeled away in celebrating his first Champions League goal in seven years, to seal Liverpool's slender 3-2 win over AC Milan. The stalwart Reds midfielder's gesture could be interpreted in a number of ways but no doubt underscores his desire to stay at the centre of all things Anfield. His recent four-year contract extension shows Liverpool's confidence in their former Sunderland star. But the 31-year-old slipped out of England's starting XI in the summer's Euros and will be itching to force his way back into Gareth Southgate's side. Boosting his goals tally will be one way to stay on the England radar, while also extending his efficacy with Jurgen Klopp's men.
City fire ominous warning
Manchester City wasted no time in venting any lingering frustrations over losing last season's final to Chelsea. Pep Guardiola's men hit RB Leipzig for six, with Jack Grealish firing a fine first goal in the competition. City were at their fluent best, albeit against modest opposition for the scale of the tournament. But the only top Premier League side searching a summer striker who failed to bring one in continue to rack up the goals. City's 6-3 triumph proved facile enough, but the match ball still belonged to Leipzig striker Christopher Nkunku, who conjured a hat-trick in among the most trying of circumstances.
Romelu Lukaku has already justified his transfer fee
It might have cost a cool £98million for Chelsea to bring Lukaku back for a second Stamford Bridge stint, but the 28-year-old has quickly proven his goalscoring prowess. The former Anderlecht youngster claimed his fourth goal in as many matches for the Blues since leaving Inter Milan, with a powerful header to settle a tetchy west London clash with Zenit St Petersburg. The Blues started their Champions League defence with a slender 1-0 win over the stubborn Russians, with Lukaku burying the only genuine chance of the night.
History threatens to repeat itself for United
Last term a shock defeat to Istanbul Basaksehir thwarted United's Champions League aspirations. This time around a trip to Swiss outfit Young Boys first up ought to have offered a comfortable easing back into the tournament. Instead United slipped to a damaging defeat, where not even Cristiano Ronaldo could save the day. The evergreen Portugal star's opener should have had United in control, but Aaron Wan-Bissaka's red card halted the momentum and Jesse Lingard's errant back-pass gifted the hosts the 2-1 win at the death. Immediate, unwanted proof that Ronaldo's homecoming alone will not be enough to catapult United to a period of dominance.
Bayern coast through an alarming culture clash at Barca
Barcelona's great heritage stands on the brink of ruin amid another chastening defeat, this time a 3-0 home drubbing by Bayern Munich. The Lionel Messi years are over and his departure to Paris St Germain only serves to underscore the genuine trials at the falling Catalan giants. Boss Ronald Koeman said Barcelona's squad could not compare to that of Bayern. Financial mismanagement remains at the heart of Barca's problems. The men from Munich could teach the overreaching Spaniards a thing or two about prudence, but the lesson would be too little, too late.
