This season seems to be proving Glen is really good at what he does. Energy, hustle, decent offense, and decent defense. He is exactly the type of player a good to excellent team needs, a sixth man type of player. Davis can fill in occasionally as a starter, and can give you 30 minutes as a bench player.
This is almost square in MLE or slightly higher range as far a money. I could see a team like Chicago, maybe even Orlando offering him multiple years in this range. Unfortunately for Davis, who knows what is going to happen with the new CBA. Bad year to be a free agent.
As far as the Celtics are concerned, a lot will depend on the new CBA (I know, not breaking news). Under the current CBA, we could always give him a 1 year deal worth well more than any other team if we won't give him multiple years. I feel the Celtics would be ok paying Davis anything any other team would, but probably not for multiple seasons.