The Brazil international became the ninth most expensive player of all time following his move from Santos in May.
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"A purchase of €57m is extraordinary," Zubizarreta told reporters, "[but] I look at the overall buy and the expectations of the business.
"If the expectation was of nearly €100m and we have paid €57m, it is an extraordinary buy."
The former Spain goalkeeper was also asked whether he thought his club's sale of 22-year-old Thiago to Bayern Munich, for a fee which could rise to €25m (£21.7m), represented good or bad business.
"I do not approach it as business, he wanted to evolve as a professional," said the 51-year-old. "We received €22m (£19.1m), with two variables that Bayern will pay us as well and €1m (£869,500) more from a friendly.
"We've got €25m, that's what I know. It is good under the circumstances."