Lionel Messi's presence on the football field is enough to reduce even the most assured opponent to a trembling mess of fear and dread.

In that respect, the Barcelona forward has a provocative trait in common with the most famous, yet seldom seen beast in Scottish folklore – the Loch Ness Monster.

Barca are in Scotland this week for Wednesday's Champions League showdown with Celtic and, while those dwelling near Nessie's supposed natural habitat 180 miles north of Glasgow are concerned with matters of the unknown, Brendan Rodgers' side know all about the formidable force they will be up against.

Messi netted a hat-trick as Barca romped to a 7-0 Champions League win at Camp Nou in November and Rodgers was in no mood for tenuous comparisons at his pre-match news conference.

Asked whether the Loch Ness Monster existing or Messi being human was more believable, the Celtic manager responded: "That's the worst question I've ever been asked."

Messi missed out on Barca's 0-0 draw against Malaga on Saturday due to illness and Rodgers knows his men are facing a different proposition if he returns to Luis Enrique's line-up as expected.

"If he plays then he's arguably the world's best player," the former Liverpool boss added.

"He's shown when he does play they are a different team.   

"No matter who plays, we know Barcelona are a very good side."