He's been poor in the sense that he's playing alongside a defensive midfielder in Wanyama who's attempting to do the same job. How many times has he played central midfield alongside Dembele this season? He's also been playing centre half, a position where he's about as good as Younes Kaboul.
I have called Dier out for being a terrible centre half, yes. But as a defensive midfielder for us? I rarely see any faults. His positioning and timing for a defensive midfielder is the best I have seen since Sandro for us, in fact he's probably better then prime Sandro. I see many faults in Wanyamas game every single week but it rarely puts us under pressure or costs us (Leicester City aside) because his positioning is usually suspect.
Like I've said, I like Wanyama. I was VERY skeptical when we signed him as I saw nothing more than a clogger. Whenever we played Southampton he was dreadful but he's come in and he's pretty much done what Dier did to a lot of us last season and made us eat humble pie with how good of a footballer he is, but he's no Eric Dier.