Real Madrid have sold two formidable full-backs to the two clubs of Milano city. Both players are delivering very good performances.
As per journalist Fabrizio Romano, The reason why Théo Hernàndez was sold is a purely technical choice as Real Madrid felt that he was not ready then. Marcelo was an untouchable and it was decided to invest in Ferland Mendy. Eventually Theo was signed by AC Milan for a fee of €20M which is an amount well spent in regards with how the French full-back has been playing since he joined.
As for Achraf Hakimi, a technical choice has its weight on why he left the club. However, the player was sold last June because he was one of the players that could fetch an important fee to be added to the budget immediately. In fact, when Inter put €40M on the table, Madrid decided to accept immediately, because the losses were enormous, so much so that Real Madrid have not bought even a player in the summer and will do the same in January.