Newcastle are in advanced talks with Villa for the midfielder, with a deal now close.
Agbonlahor told talkSPORT: "What can you do as a club when the talks are that they've offered three contracts, he's turned them down, and his contract expires in June 2027? That's two years left.
"If you don't sell now and this carries on into January, 18 months left, what sort of offer are you going to get?
"So Aston Villa are probably looking at it and saying, listen, if he's not going to sign a new contract on the terms that we have given him, then we need to sell now and make some money.
"And some fans have been tweeting me and saying, Gabby, it's a good deal. He only scored one Premier League goal last season.
"I don't look at him as a natural goal scorer, but I think he will be missed. One of our own, local lad.
"The way he can carry that ball and create chances for others as well, he'll be a big miss and a great signing for Newcastle."