Only team who’ve challenged for the league won it and they’re solid defensively, so their attackers don’t have to worry about defenders being caught out. Our attackers have to cover a lot of ground which is detrimental to improving our attacking play.
But you defend as a team, starting with the front line. You don't have to relentlessly close down players or restrict space for 80 minutes, but you have to do it as a team and it's calculated where everyone has to be in sync whenever you do it. Liverpool under Klopp do not press nearly as hard or for long as a team as they once did. Their pressing higher up the pitch results in teams punting the ball away and giving up possession. When their midfield intercepts and presses, it's typically one or two passes before the ball is played into space behind the backline for their players to chase, which is where defenses are completely out of shape and you have Mane, Salah, Firmino, and a midfield runner exploiting the voided spaces. It's why people joke and tease about Liverpool being a long ball team in that they don't mind losing the first ball, but they pounce on the second and third balls and are precise in their attacking phase while the defending team is out of sorts from clearing the initial ball. Then you have their fullbacks who are able to swing in passes behind the lines and into space which stretches the defensive shape all ways.
City have average to good CBs, but due to their pressing and closing down of space up the pitch, they are able to maintain some shape at the back and defend from inside out. If you sit back against them, then that invites a little more pressure and few possession, but if you have the players and patience, it takes only a few pass to break them down and you're into space. They have so many technical players, which is why they are able to keep possession for so long and to continue to be threatening on the ball in opposing halves.
Our attacking issues have been against teams who sit back or congest the midfield. If the opposing team wants to push up against United and allow them to be organized in a defensive shape, they'll get picked apart especially with Bruno and Pogba in the side with the pace of James, Greenwood, Rashford, and Martial. It's clear as day.
Spurs "challenged" for the league under Poch with CBs who aren't that quick or aglie, but they were rock solid and good with the ball. However under Poch, their work rate as a team and how they pressed/closed down was very impressive. It was never a free flowing attacking team, but they had effective attackers in space mixed with the necessary flair and clinical ability - Kane, Son, Eriksen, etc.
United have been good against other Top 6 teams this year and probably haven't done too bad in recent years. However, this defensive partnership is being used as a scapegoat against United's overall weakness of scoring enough goals against teams in the bottom half of the table. It's not wrong if those teams score a goal against United, but you're telling us that United cannot fashion enough attacking play and chances to score 2 goals?
The set piece delivery, set piece goals from defenders, those are lacking as well. So to continue to question this partnership IMO is incorrect. Yes, they don't perfectly pair together or there's no one very strong all around CB who can pair with any other CB and carry them. But this partnership has been good. Can it be better, yes, but until United get that one or two significantly, clearer player(s) or if Bailly somehow overtakes Lindelof, then this is what the team is rolling with.