Riedewald, one of a number of players to have been linked with a switch to Selhurst Park this summer, will reportedly cost the Eagles £7.5m to sign from the Dutch giants.
Having so far brought in just Ruben Loftus-Cheek on loan from Chelsea, former Ajax chief De Boer is keen to add "extra players" to his squad over the next few weeks ahead of his first taste of English football.
"It's not 100% done so we have to wait [for Riedewald]. Hopefully it is going to be a very short time," he told reporters.
"The Premier League is a very tough league and if you want to be a very solid club you can't count on only 12 or 13 first-team players, but normally 16 or 17. If we want to be a solid club we need some extra players."