You in theory can(pending league approval) but the player needs to have something in it for them to agree. Ohtani is a unicorn in the sense that he'll have zero issues with short term cash flow through endorsements and such, so there's no hit to his quality of life. Then after retirement he can flee California(and possibly the US entirely) and have much better tax treatment on the bulk of his guaranteed money. Giving, say, Matthew Boyd a 30 million dollar deal with 25 million deferred has much more downside for that caliber of player, and they do 'lose' money through TMV on deferrals so there's a natural pressure the other direction.