The draw sees Real now just four points ahead of Barcelona on the table.
Casillas said: "Mentality is very important. The lead we had has been reduced, but we shouldn't cry about it.
"We have a large lead, but it isn't definitive and we must continue focusing on one game at a time. We must focus on winning against Atletico Madrid. This moves very quickly and we play many games in few days. We have to go on."