The ultras group announced earlier this month that they would not attend European home games and would opt out of the home cup ticket scheme.

This is part of their protest against how the club is being run. Sutton said: "The Green Brigade should stick to their guns. I don't always agree with how they go about things, but I understand their frustration and the feeling that supporters are being ignored. [Daily Record]

"They are taking the kind of action they feel will force Celtic to take note. They are not alone either. I speak to a lot of Celtic fans who feel angry and frustrated at the lack of ambition shown in terms of Europe.

"The win against Hearts was a real positive but rattling a couple of domestic opponents doesn't change anything."

Sutton was part of the Celtic team that knocked Celta Vigo out of the UEFA Cup on their way to the final in 2003. He believes the club still has major questions to answer on the European stage.

He added: "It's only been a few days since they were turned over by Benfica. Martin O'Neill tried to put on a brave face, but Celtic were thoroughly outclassed.

"Sunday's win was much more like it, but supporters have been crying out for European success, and they have not been properly equipped to compete at the Europa level, never mind the Champions League.

"Boycotting big matches like this is a drastic measure but I can understand how they see this might be a way to get the message across."

Sutton believes Thursday's clash against Celta Vigo is a must-win for Celtic. He said: "It's a difficult game, and the Europa League hopes are really in the balance after the defeats to Ferencvaros and Benfica.

"It's a must-win, but the same questions remain about their ability to compete when they go up a level."

Despite the current optimism following the win against Hearts and being top of the league, Sutton is keen to highlight that domestic success should not be the club's only ambition.

He said: "The mood has improved after beating Hearts and sitting top of the league, but what happens if it goes badly again on Thursday? Anything could happen as there's a real Jekyll and Hyde feeling about this team.

"It looks like they will have enough to challenge for the title domestically, but that shouldn't be the limit of the club's ambition."

Sutton reflected on how Martin O'Neill restored Celtic's European reputation when he first joined the club after a long period of underachievement. But he believes that reputation has been tarnished by recent Champions League qualifier losses.

He said: "O'Neill restored Celtic's reputation in Europe when he first came to the club after a couple of decades in the doldrums. But that reputation has been hit in recent years when you look at the teams they have lost to in the Champions League qualifiers in particular.

"That's the point supporters are trying to make."

Sutton concluded by wishing the team success in their upcoming Europa League match against Celta Vigo. "Celta Vigo are not a Barcelona or Real Madrid, but they are a decent side, and they will be a difficult proposition.

"I hope Celtic can get a result that keeps their hopes of reaching the knockout stages alive, but finishing in the top 24 of the Europa League should be a minimum requirement, never mind the ultimate goal."