Please consider that a simple reading of this parable teaches something that is actually untenible.
How can it be that the 99 are " just persons, which need no repentance."
There are no such persons anywhere, ever.
So who are these 99, and to what is Jesus referring?
In Matthew 15:1-2 we read to whom Jesus was speaking: tax collectors and sinners, Pharisees and teachers of the law. These latter two groups were being critical of Jesus as he spoke.
Is it not the case that the Pharisees and teachers of the law were the ones who represent the 99? Would not they, and all who heard, have know Jesus was speaking of them as the ones "which need not repentance?" Not that they did not need repentance, but that they denied needing repentance.
The whole story is about the pruning of the vine, in which the hard hearted Jews were removed. The one sheep is then those Jews who would hear, and the gentiles as well.
As you read the following 4 stories Jesus gave, you will see this same thread in all of them, culminating in the Rich Man and Lazarus.
This is the real reason the Pharisees and teachers of the law got more and more perturbed as Jesus spoke!