The transfer window is still open, and because both coaches are experienced, they will know whether or not to sign fresh players for the following season after selling some of them to other clubs. Teams who sell players do so due of the manager's decision; if those players cannot fit into the pattern of football they want to play, they easily release them and use the ones that are available, or better yet, look for better replacements for them. If they've been doing well even after selling some players, they should be able to do the same when the regular season begins.
It's not a matter whether the transfer window is still open or not, but suddenly changing the old midfielders with the new midfielders mostly will not work. It take time like 2-3 seasons to built up, the situation is similar like Real Madrid. Kroos, Modric and Casemiro were solid midfielders, but when Casemiro leaving and Modric get older, Tchouameni, Camavinga and Valverde are the next generation who will replace them.
Currently only Valverde is really performing in Real Madrid, Tchouameni and Camavinga are still struggling.