He was the highest paid offensive coordinator in the NFL.  He was aggressively sought after and now his time with the Bills is over.  According to Adam Schefter, The Bills are parting ways with Greg Roman.

The offense needs to control that time of possession a lot better and give that defense a rest.

That being said, the defense needs to get off the field on third down.

There's a lot of work still to be done at One Bills Drive.  I don't believe this will be the last change we will see before the end of the year.