It's not an excuse - and I don't even think we have a bad squad. A few of the very wrong signings over the years (lukaku and mkhitaryan under Mourinho), but generally its just the wrong approach to big games and Mourinho not really having a set tactic that he wants to build towards. Mourinho has alwways been a manager who in the big game, just sets his teams up to counter act the opponents strengths. So he's reacting to the others taking the front foot. Willalways put him at a disadvantage as you can't build a team like that really, rather he just buys big players and tries to make it work.
If Mourinho from day one set out to build towards a team with a set style to dominate in every game, I don't think we'd have the same team as now tbh. And it is what it is, Mourinho is a top manager but he isnt as good as Guardiola and Peps style and teams he builds to play that style will usually beat Mourinhos. So no matter who we buy, Mourinho won't ever be someone who plays an expansive game against another big team, looks to completely dominate them and goes into games confident with our normal game and makes them adapt to us. It's just not him. Literally wouldn't matter if we bought Messi, he'd still have us counter attacking in those games and hand the initiative to the opponent.
Its not even a case of the squad wasnt/isnt good enough. If the roles were reversed back in 2016, I'm pretty sure that Pep would have us playing some great stuff right now just like he does with City. Hell, keep most of the same signings but trade out Mkhitaryan for Sane who Pep really wanted and Lukaku and Ibra for Jesus who only joined because of Pep and we'd have a great attacking team, playing the same way City is now. Proobably something like this:
De Gea, Valencia, Bailly, Blind, Young, matic, Mata, pogba, sane, Jesus, Martial
Would be great to watch. But it is what it is. Obviously we can still turn into a top team, but it'd be nice going into big games and looking forward to outplaying them for a change. And its all "what could have been" but instead, because of signings like Lukaku, we have to build towards a pragmatic team to stifle attacking teams rather then be an attacking team ourselves in those games.