I mentioned this frequently at the beginning of the season, people are quick to forget how we started this season. We had no attackers in the pre-season because of injuries and the WC. We were missing Matic, Pogba, Rashford, Lukaku, Lingard. Players that play a lot haven't featured at all for us this season (Darmian, Fosu-Mensah, Garner, Mitchell, Tuanzebe, Chong, Williams, Bohui).
Fitness has been our problem the entire season. We've looked knackered since December. It's partly because of the mess of a pre-season, partly because of the coaches and partly because of the players for not taking it upon themselves to stay in a better shape.
Mourinho bred negativity in public but he did make points even though how he went about making them wasn't smart. We needed more players at the start of the season, we have lacked depth and quality and this is the result.
Compare it to Liverpool who were barely missing any players in pre-season, have had a dream PL schedule (way more rest days than any other club in the league) and few injuries and it's easy to see why they have done so well and why they still have players who are sharp.
Spurs have struggled somewhat in a similar manner to us but managed to grind out results despite playing poorly at the start of the season. City simply have way more depth than all the other teams so it's not the same problem for them. Arsenal players weren't too busy during the summer so like Liverpool got a decent rest and got a good pre-season.