Celtic manager Ronny Deila criticised the club's fans after they let off flares during their 1-0 Europa League exit in Milan tonight.
Supporters were seen releasing the devices at the last 32 match at the San Siro stadium, which they lost 4-3 on aggregate.
Now the Scottish champions could face tough sanctions from European football's governing body Uefa, just days after they were fined €10,000 (£7,326) for a similar incident during a Europa League match against Dinamo Zagreb in Croatia.
On the eve of the match, Deila called for travelling fans to ensure the behaviour of a tiny minority is not allowed to damage the club's image.
After the match, the coach described the latest incident as "very disappointing" and added: "We have to get rid of this, this is is nothing to do with football.
"Again the fans were unbelievable, you can hear the fans still singing now, that is important for us but again we have to behave in the right way."
The club has previously called for what it says is a 'tiny minority' of fans who carry flares into matches that they are causing the club's reputation damage.
During ugly scenes in Croatia, police in riot gear went into the crowd where around 1,000 fans were gathered and were later accused of being heavy handed.
Celtic has suffered from several disciplinary cases caused by fans in recent years and directors had feared a tougher penalty from Uefa.
The club was heavily fined for setting off fireworks displaying an offensive banner in a game against Udinese in 2012.
A number of smoke bombs, fireworks and a flare was let off during a match against Motherwell at Fir Park the following year.
Chief executive Peter Lawwell has said the club may be forced to play a European fixture behind closed doors if the offending continues.
On Wednesday, Deila said: "It's hurting the reputation for Celtic. I believe we have the bes fans in the world. But a few have done stupid things. It's so important that stops so we don't get further penaltie sor whatever and a bad reputation for the club."
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