Brighton will host United in Sunday's finale.
De Zerbi said: "Lewis Dunk is not available. I don't think it's a big problem, which is good news for him and us, but he won't play on Sunday. I don't know the level of his injury but it is a knee issue.
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"We will have to see Tariq Lamptey's situation because after a long time out, he played 70 minutes at Newcastle and a long time against Chelsea, while Kaoru (Mitoma), Jack (Hinshelwood), James Milner and JP (van Hecke) can be available for the start of pre-season.
"They have suffered a lot of injuries, problems, but they are still a very good team. Erik Ten Hag's Ajax team has been one of the best in quality of play. For United this season it's been tough playing without a lot of important players."