Guardiola really wants to keep Bernardo Silva at Manchester City, he has contributed greatly to Manchester City success in winning the treble winner last season. City really hope the star who operates in the central area stays at the club for a few more seasons, but they cannot force it if the player wants to seek new challenges with other clubs. Barcelona offer to swap Bernardo Silva with De Jong needs to be considered, De Jong has the same character as Silva in playing his role in the central area.
Bernado Silva can play at different positions and he is very adaptive, flexible with different tactics. Pep definitely needs Bernado Silva for Manchester City's tactical operations and I do not say he will not find solutions to replace Bernado Silva but surely he will have to deal with some difficulty to replace the player either just temporarily or permanently.
De Jong is midfielder with different skills than Bernado Silva and he has his style to play with too low intensity which does not match with Manchester City style. Bernado Silva can provide more direct attacking power for Manchester City and he can infiltrate into defense system of opponent while De Jong plays too far from opponent's box.
Guardiola has just brought in Kovacic to fill the position of Gundogan who chose to go to Barcelona, Silva departure can be tricked by the player exchange offered by Barcelona. De Jong is reliable and is used to playing with a short passing style of play, I think Manchester City should consider this offer to maintain balance in the midfield.
Again I see Kovacic has very different skills than Gundogan and they don't have many similarities in their skills and playing styles. With Kovacic and De Jong as replacements for Bernado and Gundogan, Manchester City will have less flexible attacking power and even worse ability to break through opponent defense.
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.
Valverde already had a solid position in Real Madrid and only Camavinga, Tchouameni will need to train more, play better to be matured and good enough to take over positions in midfield area of Real Madrid from Kroos and Modric. Camavinga looks good and he can replace Modric but I am not sure about Tchouameni who will need to work harder to prove himself.