The Scottish giants only managed a 2-2 draw against Atletico Madrid at Parkhead on Wednesday night despite leading the game twice.
The result left Celtic rooted to the bottom of Group E and they are facing the threat of getting knocked out of Europe before the latter half of the season.
Bonner stressed that finishing third should be the realistic aim for Celtic as it would allow them to at least play the knockout stages of the Europa League after Christmas.
He insisted that getting that third spot is going to be a major challenge given the state of the group but stressed it would be a good outcome for Celtic if they manage to stay alive in Europe.
The Celtic legend said on BBC Radio Scotland: “I think realistically, the aim for Celtic should be to be in Europe after this campaign.
“After this group finishes, at the very worst it should be the third spot.
“It’s going to be tough, it won’t be easy but it’s still achievable for them and I don’t think that’s a bad thing.”
Celtic’s European struggles continued this week but they are big favourites to beat Hibernian away from home in the league on Saturday.
