Mendieta is now picking up his career at Middlesbrough after indifferent spells at Lazio and Barcelona.
The Spanish star was at Valencia for nine years and the hub of the side that reached successive Champions League Finals in 2000 and 2001.
Kasey came face-to-face with Mendieta and Valencia in La Liga with Rayo Vallecano during those two seasons.
"They were absolutely flying and he was one of the best players in Spain when I was there," recalled our American number one.
"He would play all over the place. Very attack-minded, could come inside, score goals, defend well. He was the total package for Valencia.
"I know he went to Italy, to Barcelona and he's moved around a little since Valencia and it's strange sometimes when things go so well at one club and not at others.
"But he's a very talented individual. I haven't watched him enough since I left Spain to really pass judgement on him now although I know when he was at Barcelona there was a feeling it wasn't happening for him.
"All I can say is when I saw him play a couple of years ago he was one of the top midfielders in Europe."
Kasey Keller remembers not too long ago when Gaizka Mendieta was regarded as one of the best midfielders in Europe.