You can only stretch sides if they leave space and Rashford imo isn't a winger but rather a wide forward who should be judged on goals because he has a big goal threat. Our issue has been our inability to build attacks and then sustain those attacks hence we've been ceding possession and playing on the counter attack against big teams which suits Rashford due to him attacking the space. Below i'll give you reasons why I think Grealish would improve us further.
Most big teams have their fullbacks providing the width and having someone like Grealish ahead of Shaw would likely improve Shaw's output imo. Grealish is a player who is demonstrating fantastic ball retention skills, dribbling skills, movement and passing ability whilst under pressure and his decision making is also top notch. So what he adds to the team is more creativity, more control which benefits the collective rather than the individual as far as goals/assists are concerned. Now this doesn't mean we should banish Rashford to the bench because the season is long and there's games aplenty and having a big squad is important if we're to challenge for the big trophies domestically and in Europe.