I don't know if Nzonzi can play B2B or not. If he can, swapping him and Strootman for the same price makes sense because N'Zonzi is the better player (provided Pastore plays CM, but LW suits him better).
They f'd up when they bought Cristante. I think they were expecting someone to buy Pellegrini. Then, Pellegrini as 6th choice became a luxury for them.
Otherwise, if they're relying on Cristane and Pellegrini to start and be protagonists, it doesn't make much sense. They're still a favorite for top-4, but there's a risk now. If Dzeko, N'Zonzi, DDR, or Manolas gets injured, they can be in trouble.