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.
De Boer urged his team to build on their first-half performance when they visit Go Ahead Eagles on Sunday, with the league lead not on his mind.
"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."
It shapes as being a much-needed win for Ajax, with their opponents sitting just three points clear of the relegation play-off spots.
But PSV are in control of their own destiny, and they host Utrecht on Saturday.
Phillip Cocu's side have lost just twice in the league this season, with the last of those defeats in September last year.
Memphis Depay has continued his good form.
The Netherlands international's 13 league goals is the most in Eredivisie this season, and he has struck in three consecutive games.
At the other end of the table, lowly Dordrecht desperately need a win as they host ADO Den Haag on Saturday.
NAC Breda are winless in six matches ahead of a clash against Vitesse, while Heracles travel to Willem II.
Fresh from their win over Ajax, AZ will look to continue their push towards the top two when they visit Groningen.
Twente take on Excelsior, Heerenveen meet PEC Zwolle and Feyenoord host Cambuur.
