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The Blues host the Italian champions at Stamford Bridge on Wednesday, with the visitors top of Group E and Graham Potter’s side bottom after two games.
AC Milan’s trip to west London sees Tomori come up against his former club and Gallas feels Chelsea may have made a mistake in letting him depart last summer.
Tomori joined Chelsea in 2005 and progressed through the club’s youth system where he experienced great success – winning the FA Youth Cup and UEFA Youth League in both 2014-15 and 2015-16.
The 24-year-old England international centre-back made his Chelsea debut at the end of 2015-16 but found his opportunities limited and instead spent time out on loan in the Championship with Brighton, Hull City and Derby County.
He made a total of 27 appearances for Chelsea across all competitions before joining AC Milan on loan in January 2021 for the remainder of 2020-21, with the Serie A club having the option to buy for £25m in the summer.
AC Milan decided to sign him permanently and Tomori has massively impressed for Stefano Pioli’s side, recently signing a contract extension until 2027.
‘I don’t know why Chelsea let him go; I don’t know the reason,’ Gallas told Genting Casino.
‘I have followed him since his move and he is a player that has great capacity to play as a centre back.
‘For him, it was very important for him to play, learn and gradually improve. As a young player, sometimes people think you are ready to go straight in, which isn’t always the case.
‘Defenders are not like strikers, they need more time to develop and learn. He went to AC Milan and has had a massive impact on the team.
‘He will feel very good there, so when he comes back to Stamford Bridge, he will be a little bit emotional and he will want to show the club that they made the wrong decision in letting him go.
‘It will be interesting to see how he gets on.’
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