You don't need like for like replacements. You need a set way of playing. Milner is no TAA, but he's done a job there many times. Same with Henderson and Fabinho. None of them are like for like replacements for Van Dijk, yet they do a job, because they know the role and what that role is expected to do. If you have a philosophy and a style of play that is practised over a period of time, any player that comes into the team, should know their role and what to do when. There will always be a difference in quality and skill set between different players, but that does not really matter. Ones weakness might be another ones strength.