The midfielder has hit form again at Middlesbrough after disappointing spells at Lazio and Barcelona.
Actually on loan from Lazio, Mendieta's performances have helped Boro to their first-ever trophy, winning the Carling Cup last weekend.
Mendieta was seen as the best midfielder in Spain as he inspired Valencia to successive Champions League finals.
Gus, who came up against Mendieta in his seven-year stint at Real Zaragoza, said: "He was initially a right-back at Valencia but he was switched to midfield and did really well - he was probably the best midfielder in Spain at that time.
"He got better and better and got the chance to go to Lazio, a great move and everything was looking bright for him but sometimes you make the wrong decision in football and that wasn't the right place for him to play.
"He went back to Spain and Barcelona at one of the most difficult times there, playing full-back again, not having a chance to prove his quality and, surprisingly, he came to England.
"He has showed great technique, great ability, very clever in his runs and the way he moves, gives you angles, passing, scoring - he's the complete footballer.
"You didn't know how he would settle in the Premiership but he's done well and the team has helped him as well because they try to play football, to pass and move and have some fine individual players."
Gus Poyet describes Gaizka Mendieta as the 'complete footballer'.