After the humbling defeat to Red Bull Salzburg in the Europa League, the Ajax coach says his team has never been under so much relentless pressure.
Despite leading the domestic competition by six points, Ajax was dumped out of Europe on Thursday, going down 6-1 on aggregate to the runaway Austrian Bundesliga table-toppers.
The aggregate scoreline equalled Ajax's worst in continental competition and De Boer acknowledged his team had struggled to cope with its opponent in each tie.
"Salzburg has been a very strong opponent," he said at a press conference on Friday. "We do not experience a match against (teams like) Salzburg in the Eredivisie.
"I've never seen a team put us under so much pressure."
Ajax faces rival Feyenoord in Der Klassieker on Sunday hoping to record a fourth-straight win over Ronald Koeman's side.
De Boer is expecting a tight game at De Kuip and remains hopeful of having a full squad available to him.
"Feyenoord played well in the last weeks," he added. "It can be exciting, and beautiful, and fun.
"Lasse Schone and Thulani Serero will be tested in the coming days. Then we will know whether they are in the squad."