After suffering a 3-1 defeat in the first leg of their UEFA Champions League quarter-final tie in Paris, Chelsea face an uphill battle to reach the last four of Europe's premier club competition.
Mourinho is eager to remain positive, but acknowledges his side cannot afford to concede more than one goal at Stamford Bridge.
"If you concede one goal versus Paris, or (keep) a clean sheet, we have a chance," said Mourinho after Chelsea had eased to a 3-0 victory over Stoke City in the Premier League on Saturday.
"If we concede two goals and we have to score five, we can't do the job, so it's important to defend well versus Paris.
"A clean sheet would be perfect but even if you concede one goal we will always be able to give them a fight."
Chelsea looked set to enter the second leg of their tie with PSG trailing by a solitary goal, only for Javier Pastore to find the net in the closing stages of Wednesday's meeting.
