The experienced shot-stopper thinks that his side's success in the league is not guaranteed, while saying Mourinho's men deserved a better result against Valencia
Sunday's goalless draw against Valencia at the Santiago Bernabeu has left Madrid four points above Barcelona, less than a month after they were 10 points clear at the summit.
"I believe Madrid could lose La Liga, because Barcelona are also fighting and we have not won anything," Casillas told reporters after the stalemate versus Los Che.
"We have never said that we had won it [already].
"Our rivals also have a lot to play for and we cannot solely live off being eight or 10 points ahead."
The 30-year-old goalkeeper then conceded that Madrid deserved a better result against Valencia and noted that he did not expect to see his side held to a draw.
"No-one expected a negative result at the Bernabeu, but this was a very tough and good game, the kind of match people like to watch at home," Casillas added.
"Perhaps we had more clear chances to score, but Valencia also had some [opportunities]."
Sunday's nil-nil draw at the Bernabeu was the first time since 2006-07 against Real Betis, 42 matches ago, that Madrid have been involved in a goalless draw at home in the league.