Proving that your earning potential has been negatively impacted is different than proving how much that potential has been impacted.
The admissibility of expert testimony (the expert's opinion) is complex and I'm no expert (pun intended). However, expert opinions generally have to be based upon some scientifically verifiable process, and it must lead to some "reasonably certain" or "reasonably probable" result. In other words, it cannot just be a best guess. So, if some draft expert comes in and says TSJ would have gone 10th but for the prosecution, he has to show empirically how he reached that conclusion. If it is based upon what he was hearing from GMs, then that is just hearsay fact testimony. Instead, TSJ would instead have to get the GM to come in and say that in doing their draft analysis, they would have taken TSJ at number 10 but the fact of the prosecution dropped him off their first round board - is that possible, IDK, but I would not expect that kind of testimony, though it might well be admissible. I just don't think some draft analyst is going to be allowed to give his opinion of where TSJ would be drafted.
Interesting, such a limitation one the waiver of sovereign immunity would cut off any claim above that against Douglas County. Haven't looked into it, but it may not offer much protection to the DA or the detective from being sued personally.