I think Gagliardini is an important player to hold onto. His confidence is being recovered under Conte and it makes sense to keep him to rotate him with the starting midfielders in games against the lower teams.
He seems to play pretty well when there is no pressure attached to the game and that will also give valuable resting time to whomever he replaces in that fixture.
Nice performance on the weekend though. He's not great but he's definitely good enough to be a rotation option and the reality is if he's sold he's certainly not going to be sold for a profit at this stage and whatever fee he fetches isn't going to be enough to invest in a quality replacement.
Let's make some calculations. He arrived at 17 January on loan till 2018 summer (loan fee 2m€), Then we activated (obliged to) the direct transfer, transfer fee was something like 20m€ + some bonuses (total package reaching 28m€). For the sake of the argument, let's say the fee ends up being 25m€. So we start amortizing that in 2018, with Gags having contract remaining for 3 years to 2021 => big annual amortized value, around 8m€.
Except that we extend his contract by two years in 2018 autumn to end in 2023. So his amortized value annually dropped to 5m€. So this summer his value in our books was ~20m€, next summer it will be 15m€. I'm pretty sure he can be sold for over 15m€, at least as part of a swap deal inside Italy. Though it must be said Vecino's value in our books is lower, his salary is bigger than Gagliardini's so it's more difficult to dump him to lesser clubs.
Anyway, Valero will be the first guy out in this group. And as we add one midfielder in January and probably another one in the summer we will have Barella, Brozovic, Sensi, Jan guy, Summer guy, Vecino, Gagliardini. 7 midfielders to 3 positions. That definitely leaves one out, I don't see Gags/Vecino waiting for injuries, that 7th spot will be filled with a youngster.