Mourinho: Premier League is harder to win now
The 50-year-old returned to Stamford Bridge after six seasons away managing Inter and Real Madrid, but his side have struggled to maintain their consistency.
They have been beaten by Newcastle and Everton, while Basel did the double over them in the group stage of the UEFA Champions League.
He had no such problems during his first spell in charge as the Portuguese won the Premier League in his first two seasons, however he acknowledges replicating that feat will be extremely difficult.
"The competition is different now, the teams are different now," he told the club's official website.
"Before there were just three teams fighting for the title, now there are six.
"Nobody knows what can happen. Maybe the difference will be whether teams are in or out of Europe. Maybe the difference will be a couple of points.
"It will be good fun. We have the competition that every country in Europe wants to have, but nobody has. We have a fantastic league and we should enjoy it.
"To win the title this season would be more impressive than winning it in 2005. History plays a role. To win when the club is not historically a winner is a hard step to take, so to win the first time was hard, but to win the title this season would be a bigger achievement."
Chelsea have started the season with seven wins from 12 games and currently sit third in the table – four points behind leaders Arsenal.