Boro have recovered from a spell of just two wins in their first 10 Premiership games this season for a spot in mid-table.
They have also won the first trophy in their history with the Carling Cup triumph over Bolton last weekend - having beaten us on penalties in the quarter-final.
Former Valencia star Mendieta and Dutch winger Zenden have impressed in that time.
Acting boss David said: "Zenden has done well and the management has done well with him.
"They've converted him from a wide, orthodox outside left who has to wait for the ball to be supplied to him to one who plays in-field more in a diamond, goes searching for the ball and that shows his abilities more. The management have done brilliantly with him.
"Mendieta was a fantastic player at Valencia. I saw them play several times when they did so well in the Champions League and one wonderful night, in one of the best games I've seen, Valencia beat Barcelona 4-1 and he was fantastic.
"He lost his way at Lazio and then Barca but he's come good again.
"He's a very good footballer, very strong, good ability, sees a pass, can score a goal and when you've those players in the team, you've always got a chance, especially when you have the rocks of Southgate and Ehiogu at the back. They are a strong team."
David Pleat says Middlesbrough have always got a chance of winning games with the combination of a solid defence and the attacking prowess of Gaizka Mendieta and Boudewijn Zenden.