Does an expert who is an expert in medical malpractice have to produce a declaration of opinion based on hearsay statements?

upon in forming his opinions, his declaration does not prove the existence of those facts. The expert witness has no personal knowledge of what occurred. (See People v. Sanchez (2016) 63 Cal.4th 665, 686 ["What an expert cannot do is relate as true case-specific facts asserted in hearsay statements, unless they are independently proven by competent evidence or are covered by a hearsay exception"].)