He's here because
Moyes couldn't do business on the last day of the window so Kone stayed at Sunderland.
He's under massive pressure to deliver and if he slips up Mori is in and Koeman won't leave it much longer to purchase another CB.
He was adapted by Martinez to become a CB who can play a pass. It's just about saved his bacon with the incoming Koeman, but he's not the ideal fit. There is no blend in that current CB duo and Koeman prefers that in his pairings at the centre of defence.
Koeman, IMO, is playing a diplomatic game in his early period here at the club. He knows what this llot can do if a manager upsets the apple cart. But he's not daft and he'll await until next summer to establish his stock here and then act...something Martinez should have done when his stock was high after his first season. The old guard have to go. They had their time here and failed to produce a winning attitude and silver ware. No sentiment - get them out.