Kris Commons has played just six times for Celtic in the league so far in 2016.
Commons has struggled for game-time in recent months, with Ronny Deila seemingly overlooking the attacking midfielder.
And Hartson believes that the Norwegian manager’s failure to utilise Commons regularly has contributed towards his impending exit from Parkhead.
“Kris Commons, for me, is the best player at the club. Why has Deila turned against him,” Hartson questioned.
"Why has Commons been left kicking his heels on the bench every week when he’s the type of player who could win him games? It doesn’t make any sense.”
Commons has played just six Scottish Premiership matches for Celtic since the turn of the year, with other options preferred.
Deila has seemingly been reluctant to use the 32-year-old on a regular basis, and that has caused annoyance amongst the supporters at Parkhead.
Deila has been losing support rapidly at Celtic, and the Glasgow club have now decided that they will let him go at the end of the season.
Deila’s contract is due to expire in the summer, and Celtic confirmed today that they would be parting company with the 40-year-old in the near future.
Neil Lennon and David Moyes are amongst the favourites to take over at the Bhoys for next season.