Cech, 32, lost the No 1 spot to Thibaut Courtois for Monday's season-opening 3-1 win at Burnley - prompting rumours he will be seeking a move away from the Blues before the transfer window closes.
But Mourinho said of the Czech international: "He's a top professional. He's working as always, behaving as always, being a group man as always, being a Chelsea man as always.
"Is he totally happy? I don't believe. It's not him. It's everyone that didn't start the game. This period is hard for them and for me because it's one with one match per week. The first three matches, they clearly feel that some are playing more and some are not.
"In September, we have the Capital One Cup, Premier League and Champions League. There will be three matches a week and everybody playing with rotation, resting and decisions. September is easier for them and me in the sense that everyone feels much better.
"Petr's behaviour is fantastic and that's important."
Cech joined Chelsea in 2004 from Rennes in a £7m deal and has since won three Premier League titles, four FA Cups and a Champions League.
When quizzed on any plans to further strengthen his squad, Mourinho added: "We are very calm. My club did a fantastic job, not just because they got what we need but they did it in a record time.
"For us, the market could've closed on July 31 instead of the end of August. We are very calm. For us, the countdown clock you have in Sky Sports News is no tick tock!
"This is the sixth Premier League I've started. Every match is difficult. The result in the first match was very clear. If you play at the top of your game like we did for some good periods of the game, you have a chance to win. If you don't, you are in trouble."