You need a certain skill set to play at centre half in the Premier League. It's one of the most pressurised positions in the team, something that people who are not central defenders don't always understand.
And what happens in the coming weeks and months in that position will say a lot about how Manchester United and Liverpool do this season.
Liverpool have a clear problem in how they replace Virgil van Dijk and United have a problem with Harry Maguire's status in the team and at the club.
Centre half is a specialised position. You need two mindsets at the top level and you can't just stick a defensive midfielder into the position.
The demands on a centre half have increased hugely in the modern game. It's about trying to play the ball but also to get into a space where you can receive the ball.
But you also have to transition back and be ready to be aggressive. It's two mindsets and van Dijk showed both in the 20 minutes he played against Everton last week. He was aggressive in the tackle but also switched on and ready to get the ball when needed. He was brilliant for that 20 minutes, the best he has played this season.
Then I look at Joe Gomez and while he's calm and nice on the ball, he's not aggressive enough, he lacks that aggression to go and win the ball. He's happier to receive the ball than he is to go and win it.
People think you need to play out from the back ... but you need to be able to defend.
The top centre halves are switched on. All the time.
And people like Gomez switch off in games. He's young, he will improve as a player if his concentration gets better with age, but at the moment he has too many question marks over him. He's not there yet.
Fabinho has been excellent in the couple of games he has played there for Liverpool, he is a very calming influence at the back.
But I always find that when you put in a player who is not a natural in that position, he can have a good spell and have a few games but, inevitably, over a long spell or over a season he will be found out.
Not every week, but maybe in two or three games, and those three losses can cost you the title or the Champions League.
I'd trust Fabinho at the back more than I would Gomez. Gomez has taken his style of play from van Dijk. He's very casual but he's not as talented as van Dijk and he has errors in him too.
You need a quick brain to defend at that level and I don't see him reading situations as well as he needs to. Liverpool will need reinforcements in January, so it's a matter of getting to there with what they have to stay up challenging and get through in the Champions League. But they need to buy.
Over the course of a season, the likes of Gomez will make the mistakes you don't expect from a van Dijk, and once opposition sides work out that he is too casual in possession, or his positioning is not right, they will exploit that and see that is now Liverpool's weakness.
Clubs will work out that if you attack them through the middle, play long balls and turn the two centre halves, you can put them under pressure because Liverpool have not been used to that over the last two or three years.
If Liverpool play as high up the field as they always do, it's hard to expose them, but if you get at their centre halves, make them defend for long periods, you will get them to make errors.
As a centre half, Gomez is 'alright'. But so is Harry Maguire and 'alright' is not good enough to win a Premier League.
If you look at what happened to Manchester CIty last season, they lost Aymeric Laporte for a long spell and that cost them, because they replaced him with people like Garcia or Otamendi: 'alright' players but not as good as Laporte, not top level.
Last season, United had Maguire and it was case of needing someone to partner him.
Now the question must be asked, is Maguire even good enough?
He has had a bad spell but he is clever and honest enough with himself to admit that he needs to come back and do better.
Maguire is very reliant on how the team around him plays, if left exposed he can be caught out too easily.
He needs a defensive midfielder in front of him and a defensive partner beside him. He needs a left back who is better than Luke Shaw and maybe Alex Telles is better. Having a good defender like Telles can help Maguire.
There has to be an opportunity for players who are training every day to get into the team. Axel Tuanzebe played in the win over PSG. He said after the match it was his first game in 10 months so why not give him a run in the side, if he is mobile, can win headers and can tackle?
He has to be an improvement at least on Victor Lindelof and maybe on Maguire too.
United are not blessed with great defenders so if they have a squad player who has not had the opportunity to play, now is the time for him to be given a chance.
For United, the PSG win was a huge win and the start they had to the season can almost be forgotten with the last two results.
But United's problem over the last few years has been that inconsistency.
You look at United and you try to class them as one of the top teams in England or in Europe, but they just don't have the players who can play consistently well at that level. There is always that fear that they could go and lose on a given weekend, put in the sort of bad performance which then sets them back.
With United, one defeat is a major crisis and until they get into a habit of producing good performances over three or four months, where they are grinding out results on those days when they don't play well, when they are battling all the time, only then can you take them seriously.