The Dutch youth star completed a switch from Manchester United on Thursday, penning a season-long loan deal.
He joined the list of new signings including Jairo Riedewald and Ruben Loftus-Cheek.
“Timothy is a player who has great pace and incredible physical ability," de Boer told his club's official website.
"In today’s game, you need players with strength and speed and he is an individual with that, and as well as defence, can also play in midfield.
“From knowing him at Ajax, I can remember he was always playing in the team above for his age due to being physically very strong.
"His quality attracted a number of other clubs and I was very sad and disappointed when we lost him at a young age to go to Manchester United."
Fosu-Mensah's transfer has been necessitated by limited playing opportunities under Jose Mourinho, having made only four league appearances for Manchester United last season.
He joined the Red Devils in 2014, and under Louis van Gaal in 2015-16, had eight topflight outings in his first year of elite division football.
“Under Louis Van Gaal a couple of years ago, he made his debut and I have been keeping a check on his progress and he’s ready to play games in the Premier League and that is why I am pleased to have brought him to Crystal Palace,"de Boer added.
“I am seeing him in our system as a right centre back but he can also play full back in a 4-3-3.
"It is always nice as a manager to have a player who can play in more than one position so that you get the quality across the squad.”
The 19-year-old of Ghanaian descent made his Palace debut on Saturday, playing 78 minutes of the 3-0 home defeat to Huddersfield Town in the Premier League opener.