Well that's subjective.
Barcelona have the stereotype of developing ball playing defenders. Mingueza has his positives.
You mentioned Alena, who was more hyped, but did he ever display a higher level- and consistently, at Barca, than Mingueza did last season?
So again, 5m is chicken feed for a defender who was starting for Barcelona just last season and played a good level, who was also a Spain U21 international. But has fallen out of favour. For some clubs that's called an opportunity.
We sold Sergio Akieme for friggin 3.5m. Mingueza will have suitors, again, the greater issue will be how resistive he will be to leave as these Catalan players tend to try and stay, prove their worth until contract runs out.