Messi, so often Barca's talisman, performed well below his exceptional standards as Barcelona crashed out of the UEFA Champions League to La Liga leaders Atletico Madrid in a 2-1 aggregate defeat last Wednesday.
The 26-year-old also failed to deliver when Barca needed an attacking spark in a shock 1-0 loss at Granada on Saturday - a result that did huge damage to their hopes of defending the Liga title.
"There is nothing wrong with (Messi)," he told TV3. "He is happy with the club and with his team-mates. His problem is that he has spoiled us (with his past performances).
"He is happy, training well, physically fine. He knows that football sets you challenges every week. Talks on his new contract are going reasonably well."
And after several changes in the top job in recent seasons, Zubizarreta believes he is "prepared for everything".
"To fulfil his contract is to fulfil his contract," he added. "(Martino) has said he will do that.
"Pep Guardiola told me he would not be staying in May. A year ago we began the season with a press conference with Tito (Vilanova as head coach) and the following day we did not have him. I am prepared for everything."
