You're right. after game #22 last season, the premier top scorers were Lukaku 8, Martial 8, Pogba 7, Rashford 7. Lingard chipped in with 4 and Mata and Ander with 2 each. Matic had 1, as did Alexis. Pogba went on to be top scorer, IIRC (though he was penalty taker).
So the front line contributed 24 and midfield 12 (if classing Pogba, Lingard, Mata, Ander and Matic as m/f - I can't remember whether Lingard played as a forward or midfield, tbh). 50% (only 2 goals game from defenders, one each to Smalling and Shaw).
This year, the numbers are Rashford 13 (main penalty taker), Martial 8, Greenwood 4 = 25. from m/f James 3, McTominay 3 and Pereira 1. So 25 plays 7 - 28%. The defence has contributed 2 again, Williams and Lindelof.
Not only are we missing goals from M/F, in 2017-18, the defence contributed 10 all season (albeit we had former wingers at fullback, who were high quality finishers relative to AWB and Shaw), but that has dropped off a cliff.
If you pro-rated those 10 over 22 games, it's 5.5 goals, let's say 5 and m/f had scored an extra 5 (like last season), we'd probably be in the top 4 and looking good.
PS: I know the above argument relies on extrapolating stats in a way that would probably not reflect reality, but the underlying trend is interesting.
PPS: by the laws of average, Maguire will knock in a few by the end of the season.