In terms of fit, he is the ideal signing. A front 3 with all 3 of them shooters/scorers just doesn't work and isn't balanced even if you think Greenwoods long term future is on the right wing. You need one of the wingers to be a creator. Sancho is a creator. Have him on one of the wings opposite to Rashford or Greenwood, and now you have both the top class quality and depth to rotate however you want between these players where we'll have excellent balance in the attack. Sancho creating on the right will help out Rashford on the left, and Greenwood on the right if Sancho is on the left. Greenwood isn't good enough as a right winger to make that position his own yet, and our right wing has been a constant problem this season as we've been so heavily reliant on the left wing for all build up play, so Sancho would finally give us that right wing balance for most games anyway.
Right wing has been IMO our biggest need for ages now. Multiple years of having no threat on the right. You can't have a balanced attack like that, and we have way too many games where we don't create enough (hence the amount of games we finished with 0 or 1 goals), that attack just has to be fixed. And it's not down to striker. Cavani is a very good striker, but he'll be able to do that much more when fit if we have a creator in the attack as well. Takes away some creative burden from Bruno, and gives Bruno more space as now we have a legitimate threat on both wings, stretching the pitch.
In terms of signings that would make the most instant and long term impact, there's nobody more than Sancho and Declan Rice IMO. Both of them are types of players we need in positions of need. Sancho though is a proven borderline world class player for a 3 years now in that exact position. He's a creator who will help all those around him, share some creative burden from both Bruno and what Wan Bissaka currently has (as he tends to be alone on the right, even though he has absolutely no creativity). It just balances out that right wing really nicely and compliments the players around him, so it's one of the most straightforward fits into the team and the profile of player being English and having lived in Manchester already and being 21, combined with playing naturally in a recurring problem position for ~5 years now, makes him literally the ideal signing.
That was funny. Greenwood has never impacted a game the way Rashford has, multiple times.