Dias suffered a hamstring injury against Peterborough United in the FA Cup on Tuesday and will be sidelined for up to a month.
The 24-year-old has been immense once again at the back for the Citizens this season, having played a key role in their title triumph last term.
Dias has featured 25 times in the Premier League this season and has contributed two goals and three assists.
"Ruben will be unavailable," City manager Pep Guardiola told his pre-match press conference. "Muscular. Hamstring. Four to six weeks. It happens.
"In other seasons our best players have been out for months.
"I'd love him but he's not there, I will not cry. It is what it is. We have decent players available. We have 14-15 players. With these players we are going to fight and try to play good.
"[It's] impossible to fight if you don't play good. We play with our people and hopefully our people support as they always do."
Manchester City go into the weekend six points clear of second placed Liverpool but the champions have played a game more.
