Unpopular opinion coming, but I think the time to get the next starting midfielder that should be signed is next summer- and that option should be a more creative mid than Ndombele. As good as he is, he should have been our backup option should de Jong had signed for someone else.
I tend to think that with a full summers rest and with the off the ball work of both Arthur and de Jong (goodbye Busi) that Rakitic can have a good season next year. He is the most natural offensive midfielder we have (102 goals and 108 assists in 623 games) so the two young players can liberate his duties to unleash his offensive game.
By that token Busquets should be the sacrificial lamb as his game is too limiting and static these days, something we can not afford as Messi does nothing off the ball and Suarez is ever declining (as is Rakitic in that aspect).
Now if "10 Touch Coutinho" could fulfil the Iniesta role with Arthur and de Jong then there is another option for next season for the creative midfielder. But he will be tripping over Messi feet unless Messi is strictly told not to go deep.