Oh yeah not a lot the club could do really, more just pointing out that he left a whole that hasn't been filled properly yet. We're still waiting on Morata.
Drogba, Terry and other big names for us under Roman have flirted and done even more with other clubs, I'm not so sure why such a big fuss is made when the likes of Courtois and Hazard do it. Don't get me wrong, if I was in their shoes I wouldn't do it, I think it lacks respect still no matter how strong their connection is with the club and no matter how modern football works these days but it is what it is. Drogba was adamant on more than a few occasions that he wanted to leave Chelsea, his words were even stronger than anything Costa said.
I think Costa's character is also misjudged partly, teammates and people at the clubs he's played for have always been full of praise.