The Blues beat the Premier League leaders to sign the Ukrainian in January after eyeing Arsenal, who would be his priority club, by agreeing to pay £89million for his services.
“Mudryk may have made a mistake, he got the opportunity to play for Arsenal,” former defender Gallas told Genting Casino.
“The way Arsenal play, he would fit into that team and at the end of the season he might regret it. What if Chelsea don’t qualify for the Champions League?
“Arsenal could be the champions in the end. At the end of the season, maybe he will regret it. It could have been an opportunity for him to play well and in the Champions League next season.
“However, Arsenal will have to move on, and now they will have the chance to bring in new players so they can play with the players they want.”
Gallas, who has played for both Arsenal and Chelsea in his playing career, added that alternative Gunners signing Leandro Trossard may struggle to produce his best in north London.
He said: “I don’t know if Leandro Trossard is a good recruit. He has a big profile like his teammates but the question is, can he raise his level at a big club?
“If he’s a good signing, he’ll help Arsenal win. When you bring players to a big club, they have to play very well, so we have to wait and see what happens.