The pitch seemed too big for him to be effective. It was surprisingly big since West Ham stretched play more than anyone thought they would. Consequently there were big open spaces in midfield and a lot of ground to cover for our central midfielders. Wijnaldum and the others are now masters of this of course. Their first instinct on recovering the ball or receiving it from a colleague is to shrug off the opponent's challenge and then carry the ball forward at speed. As they do so they are generally faced by our three attackers also ploughing ahead, making the pitch bigger still. In these circumstances you want your midfielders to be quick over long distances with the ball at their feet.
Keita isn't like that. I said last night that he is an excellent player who would be more suited to Barcelona than us. He's not slow exactly, but his speed with the ball happens over short distances, not long - and he has the gift of appearing to maintain that speed in areas which are crowded. Give him two opponents doubling up on him and he's likely to dribble past them both, or deliver a quick pass and move to a colleague just a few paces away. Give him the big pitch, and teammates moving away from rather than towards him, and he's redundant.
Last night I expect that Klopp thought West Ham would present just a dense, massed, low-lying defence. Keita therefore was a sensible choice. But in fact there were only a few occasions where Liverpool were faced by that kind of massed defence - mainly in the first half. I thought then that Keita looked to be the player most likely to break through. He offered his trademark incisions through the centre but none of the one-twos quite came off (usually because Firmino and Mane weren't precise enough). But in the second half, with the bigger pitch, he was an irrelevance. Oxlade-Chamberlain showed how to play the game as soon as he came on.
I'd consider him for Atletico however. That might be exactly the kind of game in which his skills prosper. A Hendo-Fabinho-Gini midfield might be a redundant one against them.
PS He wasn't helped last night by Mane's dreadful performance in the first 65 minutes.