Manchester United definitely has a better shape than Chelsea right now. If you think Chelsea's going to win against Manchester United in today's match, you need to rewatch the Chelsea vs Brentford in which Chelsea got drawn. So far in this season, Chelsea hasn't met strong teams at all and lose their shits against some weaker teams. Manchester United won against Tottenham in the most recent match, mind you. Manchester United will have better odds of winning against Chelsea.
It seems you missed the match between Tottenham and Chelsea. Chelsea outclassed Tottenham and would have won that match if not for the controversial Harry Kane goal. It is true that Chelsea was very sloppy in the match against Brentford, it has a very good team that can win against any team in the EPL. Although Chelsea would play without N'Golo Kanté, Reece James, Wesley Fofana, and Conor Gallagher. But I doubt if Manchester United would win this match. My prediction is a draw or Chelsea would win by 2-0.
He got punished for indiscipline when MU meet Tottenham which I think was very detrimental for the MU squad, especially on the attacker side. Martial is got injured, while Rushford still on not in the best performance, so we don't know how they go, seems like very risky to bet on MU for this time. better bet in safe by choose Chelsea, the team look on fire, Their coordination has started well.
Ronaldo's absence would not be felt so much. He has failed to make any important impact on the team rather he has been the main source of distraction and controversies. Marcus Rashford deserves to start the match because he has been outstanding recently. Chelsea's defenses would have to be vigilant because Rashford has the strength and pace.