Oussama Assaidi came off the bench to score a superb winner as Stoke stunned Chelsea 3-2 in the Premier League on Saturday.
The Blues' first league defeat to the Potters since 1975 saw them slip to third in the table, behind Liverpool in second and five points adrift of leaders Arsenal.
However, Mourinho feels the squad he has inherited from previous manager Rafael Benitez is distanced from the title-winning side he developed during his first tenure at Stamford Bridge.
The Portuguese tactician left out 35-year-old Frank Lampard in favour of 23-year-old German forward Andre Schurrle, who netted a brace for the Blues and Mourinho believes it is difficult to improve ageing players.
"It is my squad," he said.
"For good and for bad, it is my squad. Of course, it is not the perfect squad.
"I try to improve my players. But it is one thing to improve a player who is 20-years-old, which you can. If you ask me, can I improve Eden Hazard? Yes I can.
"Can I improve a player who is 30-years-old? How?"
Despite sitting level on points with second-placed Liverpool, albeit behind on goal difference, Mourinho believes there are no front runners in the title race.
"Who told you we are the first contender to win the league?," he quizzed.
"I don't think we are. I think we are one of the six teams that are potential contenders.
"While other teams are on top of their game, I think we are a team playing different than we did in the past."
Mourinho first arrived in London in 2004, winning back-to-back Premier League titles before finishing runners-up to Manchester United in 2006/07; but the 50-year-old has not lost sight of the fact Chelsea have finished sixth and third in their two previous top-flight campaigns.
He added: "Remember, that in the past two years, this team finished 25 points behind the winners and finished 14 points behind the winners.
"We are not speaking with me about my team being champions last year or finishing one or two points behind.
"My team in the last two or three years in December was 15 to 20 points behind the leaders and I'm saying we are building a team, a style of play with the players we have, and of course we want the best results."
Mourinho went on to bemoan the lack of goals scored by Chelsea's strike force with the front three of Fernando Torres, Demba Ba and Samuel Eto'o only mustering four between them.
"If you tell me Aguero has scored already 10 or 12 goals, Negrado 10 or 12 goals, Dzeko four or five, Rooney eight or 10, Giroud eight or 10, Sturridge eight or 10, Suarez 10 or 12," he said.
"If you ask me would I like my strikers to score eight, 10 or 12 goals? Yes. If we had eight, 10 or 12 goals, we would be top of the league.
"That's a reality, but my strikers give me everything and the team works hard.
"If players are coming now or at the end of the season, probably at the end of the season, yes, because it is more normal, but no, I am not pushing for players now."