The Scottish champions started the Europa League group game as favourites to pick up all three points, helped by Sparta Prague’s lack of competitive action in recent weeks.
However, Sparta Prague stunned Celtic by going 2-0 up by half time, with Lukas Julis grabbing both goals to leave the hosts reeling.
Leigh Griffiths pulled one back for Celtic in the 65th minute, but no comeback took place and Julis helped himself to another goal before Ladislav Krejci capped off a memorable night for the Czech visitors by scoring their fourth in injury time.
Bhoys legend Bonner was watching on and feels the result is a hammering and humiliating for Celtic.
“It’s turned into an absolute hammering at home”, Bonner said on BBC Sportsound.
“It’s been a nightmare night for Celtic and there’s no hiding from this.
“This is an embarrassing scoreline.”
Former Scotland international Michael Stewart meanwhile feels that the loss sends Celtic back to square one and raises real worries for the side.
“Sparta Prague aren’t even able to play domestically, they’re not even sure if they can train!
“This is a really concerning result and performance.
“After a positive week Celtic are back to square one”, Stewart added.
The loss means that Celtic are bottom of their Europa League group with only one point from three games and have real work to do in the coming weeks.
