However Christian Pulisic is fit again and travelled with the squad to Sweden after recovering from an ankle injury.
“Mason does not really feel better,” said Chelsea boss Thomas Tuchel.
“To stop any speculation, it’s not Covid, he’s negative for Covid. He’s just ill and he does not feel good enough to take part in training and of course take part in the trip to Malmo.”
Chelsea go into the game strong favourites after beating Malmo 4-0 two weeks ago.
Tuchel’s side have won two of their three games in the Champions League so far and sit second in Group H, three points behind Juventus.
“I think it’s the fifth, sixth game in a row where we are considered as favourites,” added Tuchel.
“I like a lot how my team approaches these matches because we put in a lot of effort and a lot of intensity.
“When you say very rightly that Malmo maybe needs an extraordinary performance then I think they will absolutely prepare for this and any team on this kind of level is ready to create an extraordinary performance, so we should be prepared.
“We should be humble and respectful enough towards the game and the opponent and anything can happen.
“Malmo also has nothing to lose, they can play with a lot of freedom, a lot of courage, they will be supported by an emotional crowd