A "promising player" is someone in sports who shows great potential and skills, indicating they will likely become very successful in their future career. Full definition
We have young promising players who can be a name after the competition. (justarsenal.com)
I also fully agree with your list of «expensive» flops and near - free promising players who probably deserve giving more playing time. (justarsenal.com)
I never really heard of Italian clubs selling promising players for cheap. (justarsenal.com)