Despite leading their domestic competition by six points, Ajax were 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 their opponents in each tie.
Ajax's only defeats in 2014 have come in Europe, and the 43-year-old suggested their domestic rivals may not have prepared his side enough for the challenge.
"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 face rivals 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."
