Honestly its trail and error.
This one was pretty "simple".
I looked for an DM/CM and Caldwell fits perfectly. I wanted him since the start of the save.
And now with the injury of Matt Polster, I really need one.
So I looked at the RSL squad and saw that they dont really have a striker.
They are using Rubio Rubin as striker and playing 442.
Like I mentioned I never had Gustavo Bou, since he was injured so I just ask for a trade.
The negotiation went like this:
Start:: Bou <-> Caldwell
Bou + Boateng <-> Caldwell
Bou + Boateng <-> Caldwell + Subah
Bou + Boateng + two first rounder <-> Caldwell + Subah
Bou + Boateng + 2nd + 3rd rounder <-> Caldwell + Subah + Bailey
Bou + Boateng + 1st + 2nd rounder + 65k GAM <-> Caldwell + Subah + Bailey
Final -> Bou + Boateng + 2nd + 3rd rounder + 130k GAM <-> Caldwell + Subah + Bailey
Also RSL got no DP player at the moment, so this helped also.
Its always like "you need ask for a trade in the right moment"
And ofcause you need to offer something good and not just backup players.