He may be good enough in strictly footballing terms (though seems he has dropped off the level he was at with us).
For me it's about the effect on the squad mentality and unity. It's clearly been a massive part of the effort to get the mental approach right and choosing carefully players that can continually push themselves every week. I'm not sure Coutinho fits that profile physically or mentally.
Perhaps most importantly the potential impact on the psychology of the squad if they see him walking back in after the hissy fit he made to get out ('right, so we need to be fully committed to this project, together, hard-working but I can also stomp out and still come back a few years later...'). Too risky.