I don't think it's a case of his legs going, it's his inability to organise the midfield when Henderson isn't in the side doing that for him.
There's a reason why our win percentage doesn't change when fabinho misses games and a reason why liverpools win percentage absolutely plummets whenever Henderson doesn't start in midfield.
If Henderson plays 6 he is a massive leader, he never shuts up organising and making sure everyone is at it. When Henderson is an 8 he still brings those qualities, a huge workrate, he's constantly covering Trent, he's in his ear making sure the workrrate never stops, that the intensity is there from everyone around him.
Fabinho doesn't lead and he doesn't organise, as a 6 he can see Elliott is out of position, he can see Trent making stupid runs that's leaving us so easy to hit on the counter with Elliott out of position as well but he's doing nothing about it, Henderson would be going apeshit at Trent and Elliott.
This page just sums it up for me, people making him blameless, saying he's the last of our problems, the guy needs to pull his finger out, with Henderson injured you organise and make it compact around him and make sure our intensity doesn't drop. He's not doing that.