Hartson wrote in the Scottish Sun: "I have to say, if I were in his shoes I would NOT be giving up the Hoops for Southampton.
"With every respect to Saints, he's going to a far lesser club, no matter if they play in what's termed the greatest league in the world.
"Listen, I wish the lad all the luck in the world. I sincerely congratulate him on making such an impact after barely two years at Parkhead that he's persuaded a club to come in with a massive £12.5million offer.
"But in my eyes he's leaving a world-renowned club for a bottom-six outfit which will be fighting relegation in the season ahead."