To be fair, they have. They lost their best players last season and didn't really replace them, too.
I think they've been missing their front three for most of the season in Mitoma, March and Ferguson. Think they've missed their two full backs most of the season, CBs in and out, and midfielders seem to have been in and out too. But they don't have backup to these players anywhere near the same quality. They've relied on the likes of Lallana, Miller and Welbeck a lot, they've played CBs in midfield etc. Losing Mitoma and March has really hurt them in my opinion. When you see the likes of Adringa and Enciso (a CM/CAM) on the wings, it's just a big drop off in ability in those positions. Estupinan also has only featured in half their games and has had a stop start season.
If you took our injury problems and added long term injury to Garnacho, niggling injuries to Hojlund, Dalot and Mainoo, and long term injury to Rashford (in form, March had 7 apps 3 goals and 1 assist before injury) then you'd have something like the injury issues they have. And they don't have the quality we do in backup players either.