Yeah, the FFP excuse doesn't really make sense. The most likely 2 options are therefore either 1) there literally wasn't enough cash in the bank for anything close to the up-front fee a decent quality midfielder would cost to buy outright. The most recent accounts showed the cash in the bank was really low and we still owe about a quarter of a billion pounds for previous transfer fees paid in instalments. This would also explain why the club waited so late in the window (hoping a player sale would raise funds), and why they turned down some bids for players they wanted rid of and accepted others (because they were insisting on more cash up front and less in instalments).
Option 2 is that the club is just as poorly run and short-term in their business as they usually are. Paying £10m for a 1 year loan vs £17-30m for a permanent move means a 'saving' for the financial year in pure cash terms, even though much like the Casemiro deal, it's a panicked short-term midfield bandage after a poor start rather than a permanent cure.