Can a contract be binding if one party signs their copy contract with the effective date left blank?

A physician signed a contract with a Preferred Provider Organization (PPO) that was supposed to steer patients to the physician in return for discounts on the PPO's members (patients') bills to the insurance payers. Instead of the PPO steering patients, they lease/rent out the physician's contracted discount rates to non-contracted payers. Would it be possible to require reimbursement of the discounts allowed under the contract because the contract that the physician signed did not have an effective date. It was left blank when he signed it. However, I'm sure that once he faxed the signed contract back to the PPO, they filled out the effective date. But the physician still does not have an effective date on his contract. Can he claim that this contract is not effective because his does not have an effective date?