Manchester City coach Enzo Maresca said on Monday finding form early in the season "was not easy" for his World Cup players ahead of the club's Champions League opener at Porto.
"I think it's correct that you see these kind of players that they start to be tired," Maresca said at the Dragao Stadium before Tuesday's match.
Maresca was responding to remarks by Erling Haaland posted on the striker's his YouTube channel earlier in the day.
The Norwegian has three goals in three league matches but did not play in warm up games, following Norway's run to the World Cup quarterfinals.
"The new season started and I didn't even have a pre-season. I normally have a bit of pre-season but this time, just nothing," said Haaland.
"The physical side will come, if you manage to stay injury free, but you need to be sharp mentally as well, and that's not easy.
"We need to have that eager feeling to go back but we don't because there is no time to properly stay away from football because there are so many games and the season is longer.
"I'm not complaining because it's still a positive thing that you get to play football throughout the whole year and live off it.
"But sometimes we need a break as well.
"I never played a World Cup so I don't know how it is."
MARESCA WAS SYMPATHETIC
"It's it's not easy for them it's not easy for Erling, it's not easy for most of the players that they continue to play for their international team," said the coach.
"It's correct to give them some rest that can be one game out or can be one week away depends a little bit in which moment we are.
"To be focused on the day by day makes it very complicated to think where we can be in February, March, April in terms of expectation.
"The most important thing is to see that we are improving game after game."