I think Matic needs a rest he's looking a bit tired and this apparently happened a lot at Chelsea he was strong in the first half of seasons then tailed off a bit, I think he's rushed back from an injury and is playing through it until others are fit which is risky but fair play to him as we don't have much choice with injuries/suspensions.
Herrera I don't think we ever that creative as a player he's always been a bit of a neat and tidy player and where he's really excelled was as a ball winner he reads the game well so at his best gets plenty of tackles and interceptions plus he's good enough with the ball to pass it off quickly but against the very best opposition which guys like Silva & De Bruyne are can he do that?
That is the problem who would want to be the backup to Pogba? Because when fit Pogba will play and will play for 90 minutes but it might be a guy like Periera who could fill other positions when he's not backup for Pogba as he can play in those positions behind the front man or even play in a 3 man midfield with Pogba.
I don't think anyone has a top class replacement for their best player. Chelsea can't replace Hazard, Spurs can't replace Kane, Arsenal can't replace Sanchez etc these guys are the difference makers in these teams. The only club who possibly could are City is De Bruyne was injured for 2-3 months then they've got Bernardo Silva who would probably come in and fill his spot not to the same level of De Bruyne but to a very high level. These other teams the backup to the top players are miles below the level of the top guy.