I have had this very recently. Drummer with no flair, questionable timing, no ability to improvise and no will to push himself technically. He was clearly holding us back so we decided to get rid. We have now got a drummer who is the polar opposite and has come in and learnt the 25+ songs for our next gig (tonight) within the space of 3 weeks. As others have said, there is an inevitability about it so probably best to make the decision now. Not an easy thing to do either. Best of luck.