I'm not concerned. I think Grant will be restricted as a FA next summer if he isn't extended tomorrow which means the Celtics can still just match if another team throws him a big bag and I'm skeptical there will be a team out there who wants Grant so much they'll offer him something the Celtics won't want to match.
For him to get a sizeable offer, he'd have to be excellent this season and if he's that good this year then we'll likely having a starting PF job waiting for him.
Someone will come in and overpay for him. Always happens in free agency. A young team will see him as a glue guy and will think that adding him will help change their defensive identity. Key role guys on good teams always get paid more than they're really worth once they hit the market.
To me the only way he's staying is if they can extend him now.
It is true that it may happen that someone comes in and overpays. But we can match. None of us know what numbers are being discussed but if extending him now means overpaying now instead of later, what is the difference?
And to overpay, a team will need cap space. Or they can do a sign and trade with an exemption if they have one. But at that point, we can either work out a sign and trade or just match, like PHO did with Ayton. It would have been nice to work out an extension, but I don't see it as the only way he is staying. There are plenty of other ways for him to stay.
Actually, the only way that he leaves is if the Celtics decide the offer is too high to match. I see that as the least likely outcome.