Sandro Tonali stopped to wave and thank Milan fans who chanted his name and sang ‘one of us’ during the warm-up for Newcastle United at San Siro.
It kicks off at 17.45 UK time (16.45 GMT).
This is the so-called Group of Death, also including Gianluigi Donnarumma’s Paris Saint-Germain and Germans Borussia Dortmund.
The Magpies are back in the Champions League for the first time after a 20-year absence, whereas the Rossoneri reached the semi-final last season.
It was above all the first time that Tonali faced his old club, having made the €80m transfer over the summer, which allowed Milan to completely revamp their squad.
As he emerged for the warm-up, Tonali was greeted with cheers and his traditional chant from the local supporters, so he stopped several times to wave and thank them.
The ultras also sang ‘Tonali one of us’ as he was getting ready to play as their opponent.
He is a lifelong fan of the Rossoneri and admitted he felt heartbroken at leaving this summer.