Chelsea received widespread praise for their performance in winning 3-1 at Burnley on Monday, when Mourinho described Spanish midfielder Cesc Fabregas as a "maestro" after his first Premier League game for the club.
But ahead of Saturday's visit from Leicester, last season's Championship (second tier) winners, the manager warned: "It's difficult. They're champions, they come with a winning mentality, high expectations and motivation to play in the Premier League."
He said he was impressed with the Midlands club's performance in their first game last Saturday, when they drew 2-2 at home to Everton, who finished fifth last season.
"They had a difficult start, but managed to equalise and gave a good game to Everton. I don't think it will be different (on Saturday)."
Asked about goalkeeper Petr Cech, who was left out at Burnley, where Thibaut Courtois made his debut, Mourinho said: "Petr's behaviour (has been) fantastic and that's important.
"Is he totally happy I don't believe so. This period is hard for them and for me because it's a period with only one game per week.
"In September, with three games per week, rotation and everybody playing, it's an easy period for them and for me."
He refused to comment on his former Inter Milan player Mario Balotelli, or the controversy surrounding former Cardiff manager Malky Mackay, but did have words of support for Tony Pulis, who left Crystal Palace last week, two days before the start of the new season.
"I hope Pulis comes back to football quickly because he did fantastic work and deserves to be in the Premier League as soon as possible," he said.