Let's look at the statistics from WhoScored, who gives players points during a match based on their performance, and gives an average based on all performances during a season. A short explanation of how the stats are calculated here:
https://www.whoscored.com/Explanations
A short Youtube clip with an explanation too:
Sure, it's not proof that one player is better than another, but it should give us a pretty good indication, yeah? So here are our players average stats this season. Someone like Gazzaniga, who's had one good match against Palace will obviously score higher than a lot of players who have played many matches and so on.
Here's an excel table I just made (I couldn't find this list on whoscored for some reason, so had to compile it myself), of every player that has played for us this season (bar Walker Peters, who's barely played two matches?):
Sissoko scores just above Winks (which surprised me, to be honest), Wanyama who's barely played this season, Lamela who has barely played, and Llorente who has barely played. Dembele, who has also gotten a lot of stick this season, and many want to replace, scores a little higher than Sissoko, but apart from that group of players, there's a pretty big jump from Dembele at 6.64 to Lloris at 6.83. The total score of our team is 146.04, divided by 21, is 6.95. So the team average is 6.95. If we take Kane's ridiculous score out of the equation, we get an average of 6.90.
So! Sissoko is scoring way below average, he's got 16 players ahead of him, that's performing better than him in the team this season, and the players below him, Winks, Wanyama, Lamela and Llorente, have all had very limited playing time, three of them due to injury.
Should he really be playing when everyone is fit? It sure doesn't look like it. Has he been one of our most important players this season? Absolutely not! There's just nothing here to suggest that there are any hidden attributes he adds to the team that we can't spot with the naked eye. There aren't any stats to prove it, anyway (these stats certainly don't, anyway).
Oh, and by the way, I'm by no means a stats geek, I can't explain all the data involved in the calculations, but I think it's interesting to look at none the less. Most of the numbers here I think more or less support how I've perceived various player's performances this season. I reckon Trippier scores quite high due to his assists stats, for instance, and I find it a bit strange that Son doesn't score higher, and that Winks is below Sissoko (though Winks had a few poor matches after that great game against Real).