The striker signed for the Toffees from Brighton on Friday.
However, because of the Bank Holiday weekend, Maupay - through no fault of his own - did not meet Premier League requirements to feature at Leeds.
Speaking to BT Sport ahead of the Elland Road clash, Lampard said: “We registered Neal on Friday afternoon at 4pm but the Premier Leeague have not been able to register him.
“There must be one working day to be registered. We’ve managed to get three non-working days, four days of him being our player and I wish I had the non-working days the Premier League had because it’s amazing we can’t register him - but I’ll explain more later.”