He's not as prolific as he was a few seasons ago but one thing that cannot be faulted is his all around game. He's dropped off into space at the right time, has well timed runs off the opposition backline, his movement has really come into a place of maturity this season. I feel the team plays better when Rashford is upfront as opposed to Ronaldo. They make better decisions the feeling in attacking phases is to get it into Cristiano as soon as possible whereas when he's not playing the ball is recycled in possession more effectively. It's not a coincidence that when Ronaldo started against West Ham all of a sudden the possession was horrendous there is clearly a factor in his demand / presence that negatively impacts the play.
It's similar to G.Jesus, if Ronaldo played for Arsenal he undoubtedly score more but when it comes to influencing the play G.Jesus is fundamental to the team bringing Martinelli / Saka into space. Similar to Firminho these players are nuisances despite not being clinical.
The real issue is that the wide players especially Sancho aren't doing enough to score goals and that's why United have a small positive goal difference compared to the rest of the top 5.