Suarez moved to Camp Nou from Liverpool in July 2014, but Aguero was top of the Catalans' shopping list before Manchester City rejected their approach.
Zubizarreta told Marca: "We had some options. The first was not Suarez. He knows it. In winter we considered the signing of Aguero. That did not work out.
"In this situation, there was no doubt: Luis Suarez was our only goal."
Suarez's move to Barca was complicated by the four-month suspension he received for biting Giorgio Chiellini during Uruguay's World Cup clash against Italy.
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However, Zubizarreta believes the incident was a blessing in disguise as rivals for his signature hesitated in making a move for the striker.
"During the negotiation process the incident with Chiellini occurred," he added.
"The issue created a seed of doubt for other clubs who wanted him. They questioned his character. I thought it was the perfect time to accelerate the process of signing him.
"I liked everything about him: he was not static, knew his way around the outside of the box and gave us a profile which we did not have.
"From the outset his willingness to sign for Barca was huge."