Kruzen has worked as an assistant to Peter Bosz since his Maccabi Tel Aviv days, following him to Ajax, Dortmund and Bayer Leverkusen. In an interview with Goal's Jochen Tittmar and SPOX, he opened up on what went wrong at the Signal Iduna Park and how they tried to move for De Ligt:
"Maybe one [the club] should have kept a little more calm. They [the Dortmund hierarchy] saw that the style they wanted to play could work [under Bosz and Kruzen], especially after the great start. Of course, there were also personal reasons.
"When Lukasz Piszczek got injured, we had to make a lot of changes in the defence. Maybe we should have pushed for a high-profile new signing that summer.
"Unfortunately, when we had that difficult phase, as is often the case in professional football, we did not have the time to sort it out and to get new players in in the winter.
"We were already in talks with some of them, and those players were of a high calibre, even for a club like BVB. I don't know whether they would've definitely signed or not, but one of them went from Ajax to Juventus for a lot of money this summer."
Kruzen also placed De Ligt among the most talented players he has ever worked with, alongside Kai Havertz, Hakim Ziyech and Frenkie de Jong.
Bosz coached BVB for just five months in 2017, before he was replaced by Peter Stoger following a poor run of results.
De Ligt made his senior debut for Ajax during the 2016-17 campaign, quickly establishing himself as a star of the future. The centre-back started the UEFA Europa Cup final against Manchester United at the end of the season, despite being just 17 years old.
Transfer speculation soon started to surface, but he wouldn't leave the Netherlands until the summer of 2019, signing for Juventus.
Despite being the club's marquee signing, he started their first Serie A match of the season, against Parma, on the bench.
That came as a surprise to him, per AD (h/t Football Italia):
Juventus won 1-0 thanks to a goal from star defender Giorgio Chiellini, who paired with Leonardo Bonucci in a shutout performance:
Dortmund purchased another talented youngster during the 2018 January transfer window, investing in FC Basel's Manuel Akanji. The 24-year-old has developed into one of the Bundesliga's best centre-backs and is Mats Hummels' regular playing partner.
In Dan-Axel Zagadou and Leonardo Balerdi, BVB have plenty more young talent at the position.