I realise I sound like a patronising twunt when I say this but.....you cannot judge how good defenders are without looking at the system they play in.
Lascelles plays in a low block defense (essentially they play very deep and crowd out space, looking to allow the opposition lots of shots but from bad positions in crowded areas). Keane also played VERY WELL in a system like this.
If we go for a high defensive line, like all the teams who go out playing to win play, then the type of centre back you need is very different. Suddenly pace and the ability to pass are far more important.
If Lascelles and Keane played together for a low block team they'd be great. For us, playing a high line, it could be a disaster.