Despite their 1-0 win over Legia Warsaw in the UEFA Europa League on Thursday, De Boer was frustrated by a sloppy second-half performance.
While Willem II are stuck in mid-table with 11 games to go, De Boer still sees them as a threat who will punish any slopiness from the champions.
"Willem II has proven to be a good footballing team," he said. "They are where they should be.
"We must ensure that we are focused and leave with a good result from Tilburg.
"Willem II plays nice football and there is usually a good atmosphere in the stadium. We certainly do not underestimate them."
Ajax go into the match 12 points behind leaders PSV and needing their fierce rivals to slip up if they are to have any chance of winning a record fifth consecutive title.
De Boer is sweating on the fitness of Davy Klaassen and Thulani Serero ahead of the trip, while Viktor Fischer is still a few weeks away from a return.
