Your conclusion is that Sissoko is not a beast because you seem to have a regen that has probably some amazing stats. I just say that bringing up a regen is not a valid argument to judge a regular player by. Especially not since Sissoko is one of the top 5 young box-to-box midfielders in the game. He only has a bit low POS and TACK, which are trainable attributes.The OP is 4 seasons in... Of course I would throw in the word regen now... I wouldn't say that if the OP is in the first season right? Lol....
I know the OP is 4 years in. So I am not mentioning any options that are 23 or older at the start of the game. Buying a 27y old leading midfielder is the max I would do because 5 years later (maximum years on a contract) he is 32 and that is about the last chance you get to sell a player for serious money.
What does interest me in this topic is that nobody else but me mentions Romulo. Why is that? He is young, has great stats in defending, attacking, mental and physical aspect of the game and is also natural fit and not injury prone. So you can use him as long as you have used Gerrard.