This is the view of expert Stan Collymore, who believes that Guardiola should have led his side to European glory so far.
The city has not yet reached the Champions League final despite ample domestic success.
Guardiola led his side to the quarterfinals last season, but the Premier League team, Liverpool, has ruled City out of the game.
The Spaniard won the Champions League as manager before, twice driving Barcelona in the title.
Mourinho has also raised the trophy twice in his career, once with Porto and once with Inter Milan.
He has also achieved European success with Manchester United, elevating the Europa League in 2017
And with the riches that the city has at its disposal, Collymore believes that Chelsea's former head would have led Etihad clothing and continental success.
"Today the big clubs – especially the big names abroad – see the sign of success winning the Champions League," writes Collymore for the Mirror.
"And as long as we love the expansive, aggressive philosophy and purity of football, if you leave the city five or six years without winning the crown, you will not be able to keep it from the soccer elite.
"I suspect that if they were given similar resources, all top executives who compete against home and abroad, including Mourinho, could already have won at least one Champions League if they were responsible in the city.
"With a banker from Sheikh Mansour, I would say that Guardiola will have to win the Champions League three times if it's six years, which is a fair move.
"That would surely bring him to the big ones of all time.
"If not, the halo will slip."
Guardiola has recently voiced doubts about Manchester City's potential in Europe.
"Manchester City can not be considered the favorite for the Champions League," he said.
"The maximum that the club has made in the competition is semi-final, I do not know if we are ready to be honest.
"When you think you can win, then you have to do it."