Five Premier League clubs are in position to be in the knockout rounds, with Manchester City and Tottenham already qualified and Chelsea, Liverpool and Manchester United all in the driver's seat to make it through the group stage.
Despite the strong showing in group play, the Man United boss thinks that the winter break taken by most leagues while the English football calendar loads up fixtures around the new year will keep English clubs from becoming a truly dominant force in Europe.
“I always say the Champions League only starts in February, and in February the English teams are [still playing] after December and January where we can play 20 matches over two months. The Germans, the French, the Spanish, the Italians, they all come from a winter break," he told the Mirror.
“So I think they arrive in better condition than the English teams."
While the fixture congestion in the winter months is one cause for concern, Mourinho believes that the higher level of competition in England adds even more stress to the squads who are trying to compete on all fronts.
“It’s also a fact that any match you play in this country, if you don’t go strong, you lose," he continued.
“We go to Bristol (City of the Championship next Wednesday) in the (EFL Cup) quarter-final and if you don’t go with a strong team, you lose. That’s as simple as that.
“In other other countries, between the top teams and the others, there is huge difference and not just in terms of talent and potential but also in terms of mentality.”
“Here, the smaller teams want to win, want to compete, want to have the courage to try and get a result.”
Mourinho's Manchester United side return to Premier League action on Saturday against Newcastle before taking on Benfica midweek.
His squad has been tested so far this season, with injuries to Paul Pogba and Zlatan Ibrahimovic taking away two of his top options, though both are pushing for a return in near future.
Defender Marcos Rojo also made a return after seven months out, playing 45 minutes for the club's under-23 side on Wednesday.