It's mostly their midfield being an epic fail. Dier good DM but doesnt offer much else, Wanyama walking injury, Winks walking injury and should probably be a sub as he is not the finished article. They had some 18 y/o academy player playing in midfield in some games, simply because they had no-one else. The only one that has had a decent season in midfield is Sissoko. They never replaced Dembele who whilst a great player, was not in Modric's league.
Provided they splash out some actual money, although this is Spurs - Levy we are talking about. And purchase an upcoming star playmaker they should be fine. But he will not come cheap, rumors they are in for Lo Celso to replace Eriksen. But with Spurs you can see this devolving into an undervalued final minute of deadline day bid for Maddison after not getting their main target Lo Celso. And then being refused by Leicester, then promoting some 18y/o academy player no one has heard of to be Eriksen's replacement.
Personally I think they need two big signings, an Eriksen replacement, if he leaves, and another top class CM. Or at least a wonderkid who might be the next big thing in CM.