Mendieta is an interesting case - as a young player he was dismissed as enthusiastic but limited in a lot of quarters until Ranieri took over at Valencia. His peak years were from the age of 24 to 27, which isn't so unusual. Admittedly, his career tailed off, although part of the problem was the moves he made - he joined both Lazio and Barca when they when on a downward trajectory. Worse, in Italy, he was asked to help fill the gaps left by Veron and Nedved, a task for which his skillset was plainly unsuited. Nor was he a match for Van Gaal's style.
It's a pity he didn't come to England in 2001 - as it was, despite waiting until he was 29, and no longer the force of old, he served Middlesbrough well, helping them clinch their first and only major trophy, until injuries took their toll on him.