Messi's goal haul in the defending champion's 7-1 rout of Bayer Leverkusen on Wednesday night was a competition record, and enough for Guardiola to rank his player beyond anyone who has gone before.
"The throne is his," he said. "Only he will decide when to leave it."
Barca teammate Cesc Fabregas echoed his coach's praise, labeling the Argentinian as "the best player in [soccer] history ... we've never seen anyone like him," while Wayne Rooney, watching from England, found the magical display so sublime it became ridiculous.
"Messi is a joke," the Manchester United star tweeted. "For me the best ever."
Guardiola added that he was just grateful to be in the presence of such greatness.
"I'm lucky to coach and manage this player," he said. "He is the best. There's no one else. He is a unique player because of his innate talent and also for his competitiveness. We will not see another player like Messi. It's not easy to score five goals, and one day he will score six."
The man himself, though, deflected talk of his performance Wednesday to the evening's greater goal.
"The important thing is that we have qualified," Messi told TV3. "The important thing is how we played as a team and we are into the next round because at this stage of the competition everyone is tough."
The three-time FIFA player of the year scored in the 25th, 42nd and 49th minutes for a 3-0 lead and his 17th career hat trick for Barcelona, then added goals in the 58th and 84th minutes for the first five-goal game of his professional career.
Messi scored his first goal when Xavi Hernandez's long pass sent him in alone, he took a touch and chipped goalkeeper Bernd Leno with his left foot from 12 yards. On the second he cut across from the right after a pass from Andres Iniesta and sent a left-footed shot in from 14 yards through a crowd of defenders.
He scored the third with a chip from 9 yards off his weaker right foot. Cesc Fabregas made a pass from just past midfield and Messi held off defender Daniel Schwaab as he took three touches.
He became the first player to score four goals twice in the Champions League — he accomplished the feat against Arsenal two years ago — when Leno batted a ball knocked toward the goal by a defender right to Messi, who poked the ball in with his left foot from an angle.
Messi's fifth was on a curling left-footed shot from 19 yards.
He has 48 goals for Barcelona this season, including 12 in the Champions League. He increased his Champions League career total to 49, tying Alfredo Di Stefano for fourth on the European Cup/Champions League career list.
"I can't remember the last time I scored five goals, so I'm very happy," Messi said. "It's nice to score five goals."
Just 24 years old, he has 228 goals for Barcelona and is seven from tying Cesar Rodriguez for the club's career scoring record. Messi has seven hat tricks this season.
Even the competition was in awe.
"Without Messi, Barca is the best team," Leverkusen coach Robin Dutt said. "And with him, they are in another galaxy."
The last time a player scored five goals in European soccer's top club competition had been when Ajax's Soren Lerby did it against Omonia in the old Champion Clubs' Cup in 1979-80.
Wednesday's victory completed a 10-2 aggregate victory over the German side to send Barca through to the quarter-finals.
Based on reporting by Newscore and the Associated Press.