The former Celtic striker feels the Norwegian is not good enough.
Indeed, the Bhoys continue to drop points under the Norwegian at a rate that is totally unfamiliar for a side who have regularly been Scottish champions.
Celtic's chances of winning the Scottish Premiership are under genuine threat with Aberdeen just four points off them in the title race.
And Hartson feels Deila isn't up to scratch at Parkhead - hitting out at those who 'lambasted' him for saying it ages ago on Twitter:
Yes Ryan and got lambasted for saying it! https://t.co/TJJg5mlY5S— John Hartson (@JohnHartson10) March 3, 2016
In response, Celtic fans generally agreed with the ex-striker on social media.
However, one made the point that pundits such as Hartson and Chris Sutton were quick to undermine the coach from the start.
Had they backed him, could he have done a better job with so many ex-pros willing him to succeed rather than fail?