The Eredivisie champions have lost at the last 32 stage of the competition in the past three seasons, and the head coach is keen to break that pattern this time around.
Having dropped out of the group stages of the Champions League earlier in the campaign, a first European title since 1995 will have to come via the Europa League.
De Boer has got his sights set on achieving that this season, but he knows Legia pose a real threat to their hopes.
"For us the Europa League is very important," said De Boer. "We want to get as far as possible.
"Legia is a very experienced team, their average age is 29, I think. They only have a couple of young guys.
"They play fairly direct football, with wide players who have much depth. They are a very good team."
The Amsterdam ArenA hosts the first leg of the tie on Thursday, before Ajax travel to Poland for the second leg on February 26.
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