It was also Celtic 's heaviest ever defeat in the competition .
Messi , 29 , became the first player in European Cup history to score six hat - tricks as he bagged his 40th career treble for club and country in Barca 's biggest European Cup win .
Barca was also Celtic 's heaviest ever defeat in the competition .
Messi , 29 , became the first player in European Cup history to score six hat - tricks as he bagged his 40th career treble for club and country in Barca 's biggest European Cup win .