De Boer's men are winless in three league matches and have lost two in a row, seeing them slip 12 points behind table-topping PSV.
Losses at Vitesse and at home to AZ have been a huge blow to Ajax's chances of winning Eredivisie for a fifth straight season.
Aron Johannsson's second-half goal, after Ajax were reduced to 10 men due to Niklas Moisander's red card, led AZ to their win.
"One point from three matches is not enough for Ajax," he said.
"At this time, we should therefore not look at the lead, but first focus on our own game.
"Ajax should build on the game in the first half."
Phillip Cocu's side have lost just twice in the league this season, with the last of those defeats in September last year.
The Netherlands international's 13 league goals is the most in Eredivisie this season, and he has struck in three consecutive games.
