Despite being unbeaten in seven domestic games, new Chelsea manager Thomas Tuchel knew he had not been really tested until he took on some tough opponents.
Chelsea's good run in the Premier league had been against teams they were expected to beat and they hadn't been particularly inspiring.
After a sloppy showing against in a 1-1 draw with Southampton in the Premier League last week, Tuchel couldn't have asked for a more demanding test of his credentials than Tuesday's Champions League clash with La Liga leaders Atletico Madrid.
The game could not have gone much better, albeit on a neutral ground in Bucharest owing to the Covid-19 situation.
It was a convincing performance from the Blues and thanks to a brilliant goal from Olivier Giroud, they take a 1-0 lead to the second leg at Stamford Bridge.
"I am super happy for my players," said an ecstatic Tuchel, but he warned "we cannot get lazy".
He knows the job is not done and you can be sure that Diego Simeone's team will pose a threat in the second leg, especially as Chelsea will be missing the suspended Jorginho and Mason Mount who have become key players.
The crowded fixture list means there is no let-up for Tuchel.
Tomorrow, Chelsea host in-form Manchester United and next Friday at Anfield take on a Liverpool side determined to get back to winning ways.
These demanding matches will be followed by games against Everton and Leeds, not easy opposition.
Despite his success so far, the verdict is probably still out on Tuchel.
Like his predecessor Frank Lampard, he has yet to get his costly superstars to gel. There have been occasional bright moments, but there has been a distinct lack of incisive play.
While Chelsea have dominated possession in most matches they do not always make the pressure count, as in their recent game against Southampton.
Early on Chelsea threatened to overwhelm their opponents but it was the Saints who took the lead and in the end Chelsea had to settle for a draw. This is not what Chelsea owner Roman Ambramowich has in mind.
Tuchel cuts a curious figure on the touchline. He is always animated and gets very agitated if things don't go his way. It will be interesting to see how he reacts, if things go badly wrong.
His man management came under scrutiny when he put on Callum Hudson-Odoi in the second half against Southampton, but then substituted him after just half-an-hour because he "didn't like his body language" -- the manager feeling the winger/wing back wasn't pulling his weight.
This was humiliating for the Chelsea youngster who understandably looked furious, particularly as he was no worse than the rest of the Chelsea team.
Chelsea fans were pleased to see Hudsion-Odoi was selected for the Atletico game and more importantly he performed well and seemed happy, although that substitution must still hurt.
Hudson-Odoi should not take that puzzling substitution too much to heart because he is not the first Chelsea player to suffer such an indignity.
By coincidence, it was also in a match against Southampton in Oct 2015 when Jose Mourinho was manager of the stuttering defending champions.
Nemanja Matic came on after half-time replacing Ramirez with the score 1-1.
The introduction of Matic only made matters worse as he was quite awful and after Chelsea had gone 3-1 down a desperate Mourinho substituted his substitute in the 73rd minute, with Loic Remy coming on for Matic. The match finished 3-1 to the Saints.
Another member of the select "Substituted Substitutes Club" is Mario Balotelli playing for Manchester City against Liverpool in April 2012.
Manager Roberto Mancini brought on Balotelli to replace the injured Carlos Tevez in the first half.
But Balottelli was so poor he was replaced in the 83rd minute by Nigel De Jong. City supporters were actually surprised Mancini let Balotelli last as long as 67 minutes.
Perhaps the most unfortunate substitute to be substituted was Aaron Lennon in the 2006 World Cup quarter-final between England and Portugal.
The 19-year-old Lennon came on for the injured David Beckham in the 51st minute and put on an excellent display.
However, as the game headed for a 0-0 draw and a penalty shoot-out, Lennon was substituted for Jamie Carragher because of the Liverpool player's experience in taking penalties.
Although Carragher scored with his penalty kick, it had to be retaken as the ref had not blown his whistle. Carragher's second effort hit the bar and Portugal went on to win 3-1 on penalties.