Chelsea coach Enzo Maresca said his players were still trying to adjust to his style and tactics after they lost 4-1 to Celtic in a pre-season friendly in Indiana on Saturday.
The flags, which numbered in the hundreds, were deemed to be "provocative messages of an offensive nature", UEFA said in a statement
The 57-year-old Australian looks set to be named Tottenham Hotspur boss, having won five trophies in two seasons at Celtic including the Scottish treble this year.
As football fans around the world united to mourn the death of Queen Elizabeth II, one club stood out. Tempers flared when Celtic F.C took to the field.
Celtic manager Brendan Rodgers hails his players' "incredible achievement" as they equal the club's own 100-year-old British record of 62 domestic matches unbeaten.
Bayern Munich ease past Celtic 3-0 in the Champions League to hand coach Jupp Heynckes a victorious return to the competition he won with them in 2013.
A Jose Callejon goal help Napoli ease into the Champions League group stage, while Sevilla also went through and Celtic suffer a scare in Kazakhstan before winning their play-off tie.
Celtic manager Brendan Rodgers hails his side's outstanding display as they moved to the verge of the Champions League group stages with a 5-0 thrashing of FC Astana in Glasgow.
Scottish champions Celtic should not have too many problems on the pitch with Northern Irish title holders Linfield in their Champions League second qualifying round first leg on Friday, but they expect a hostile reception.
The dazzling Lionel Messi returned from illness to score yet another double to help Barcelona ease into the knockout stages of the Champions League for a 13th successive year with their 2-0 win at Celtic on Wednesday.
Raheem Sterling netted at both ends as Manchester City were made to fight for a 3-3 draw with Celtic in a Champions League thriller on Wednesday, while Atletico Madrid once again did for Bayern Munich.
Celtic manager Brendan Rodgers says his side's confidence is high ahead of their Champions League clash against Manchester City following their thrashing of Kilmarnock.
Lionel Messi, Luis Suarez and Neymar combine to devastating effect as Barcelona swatted Celtic aside 7-0 at the Nou Camp to lay down a marker in Champions League Group C
Two Palestinian charities have picked up an unexpected financial windfall with £45000 (52000 euros, $59000) raised by supporters of Scottish Champions Celtic.
Celtic are beaten but not disgraced as they warm up for their crucial Champions League match against Astana with a loss to Barcelona in Dublin.
Scottish giants Celtic crashed to a 1-0 Champions League defeat to Lincoln Red Imps on Tuesday with the goal scored by Lee Casciaro, a policeman in his day job.