Kruzen has been assistant to Peter Bosz at Maccabi Tel-Aviv, Ajax, Dortmund and now Leverkusen, with the pair's tenure at BVB cut short after just five months at the helm having taken over from Thomas Tuchel.
And in discussing what went wrong, Kruzen suggested that too many players did as they pleased and that a failure to be able to recruit effectively didn't help the situation.
He told Goal and SPOX: "Back then, at Dortmund there were too many players who lived in their own world a bit. If too many players think they can do what they want, you have no chance as a coach.
"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 spoke about the future of Leverkusen star Kai Havertz, who has been strongly linked with a move to several top European clubs.Getty
The 20-year-old midfielder scored in his side's opening-day Bundesliga victory over Paderborn and Kruzen accepts that he's likely to move on to one of the elite sides in the future.
He said: "I think Kai is definitely a player for the big clubs [to look at signing] in the future. He has a lot of talent and has everything for an outstanding career.
"He scores goals, assists them and can handle the ball very well. What he can still improve on is the defensive phase, the transition - that he doesn't just stop but goes straight back. But he will also learn that.
"He has to become more of a leader this year and to take on greater responsibility. If he plays as well as he did last season, then the chance he'll leave isn't any lower. That would be fine, because then maybe he would take the next step in his career at one of the very big clubs. But let's see - in any case, Kai feels right at home with us at the moment."
And on whether or not Havertz is the best player he’s ever coached, Kruzen added: "One of the best, yes. But I would also name Hakim Ziyech, Frenkie de Jong and Matthijs de Ligt at Ajax in this context. And it is a pity that we could not work with Marco Reus at Dortmund because he was injured.
"He and Mario Gotze loved our style of play. These are all players who think not only about the next pass, but also the next situation in the game before they even get possession of the ball."