Zilliacus was keen to bid for United when majority owners, the Glazer family, originally invited offers three years ago, though was squeezed out by Sheikh Jassim and Sir Jim Ratcliffe, who eventually took a minority stake in the club.
Zilliacus told the Mirror: "I mean under Sir Jim Ratcliffe it hasn't been a roaring success. I don't think anybody could claim that. So no criticism of him. I'm sure he has done the best he can. But unfortunately, in my opinion, there have been some poor choices regarding players. The new manager, it is maybe a bit early yet to say whether he can turn the ship around.
"It's not, of course, just the manager but the squad that he has… A lot of money has been put into the club and we still haven't seen the results. I think that also reminds us of another very important thing, that this is a team."
On improving the team, the Finn continued: "I mean, football is at the end of the day a pretty simple game.
"It's about scoring more goals than you let in. You need to have a good attack, you need to have a good defence and in between that you need to have a good midfield."
Every position can be improved
He also said: "I think that I would honestly look at all parts of the team, I think that while there are some very good players, there is always room for improvement. But it is not about going out and buying the biggest star that is around.
"A good example, although he's already over the hill, is Neymar. Even at his peak, I would never have brought a player like Neymar to Manchester United because that's a player who expects everybody else to play for him.
"I would build a club very much on the basis of young, strong players who already have proven that they are really good top-class, but who still have a hunger to prove themselves in order to go even further.
"That should be the backbone of the team. Then you can have a few experienced guys in the gel as well. I think it's always good to have a mix because the more mature guys, the more experienced guys, they also know how to act when things don't go well."
Indeed, Zilliacus added: ":Look at PSG. Ousmane Dembele is such a perfect example.
"Barcelona were pretty happy to see him go. Now he's got the Ballon d'Or and he's the star player in a team where he is allowed to play the way he should. But I also think the manager has managed to get him to understand that if he wants to be a star, he has to work. And now he's doing it."