Right-back: Trent Alexander-Arnold (Liverpool)
Alexander-Arnold is undoubtedly one of the top young talents in the Premier League, let alone in terms of right-backs. To have achieved some of the feats he has at such a young age speaks volumes about his footballing ability, not to mention the fact he's a starter for one of the best clubs in the country. The likes of United's Antonio Valencia or Ashley Young might have been competition for this spot in their heyday, but they're older now and certainly nowhere near AA's ability.
Centre-back: Joel Matip (Liverpool)
Joel Matip has been in fine form for the Reds in recent weeks, and pairs up well alongside van Dijk in the heart of Liverpool's defence. He's tall, he's strong, and he's got a fair bit of pace to him, which makes him a nightmare for strikers to try and get past. I did consider putting Chris Smalling in this spot, but he has shown signs of slowing down recently, so Matip deservedly gets this choice.
Centre-back: Virgil van Dijk (Liverpool)
I imagine that once you saw Matip take up the other centre-back role, you knew who his partner was going to be. Virgil van Dijk was a no-brainer choice for this spot. He's one of the best defenders in the world right now, and could easily become the best in the near future.
Left-back: Andrew Robertson (Liverpool)
The decision to put Andrew Robertson in at left-back means that the entirety of this defence is from Liverpool. What this says about Manchester United is up to your interpretation. As for why Robertson got my pick is pretty obvious, like van Dijk, he's one of the best defenders in the world right now - certainly one of the best in the Premier League. Not only does he possess speed, but he can whip in crosses, execute killer passes, and even strike on goal when space opens up.