Haaland scored five goals as City steamrolled their German opponents into submission in a 7-0 rout to put them into the Champions League quarter-finals for the sixth consecutive season before Guardiola substituted him in the 63rd minute.
No player in Champions League history has scored six goals in a game.
"If he achieved this milestone, the record, at 22, will be boring, his life, so now he'll have a target to do it in the future, here and everywhere," Guardiola said. "So that's why I make a substitution."
Haaland became the third player in history to score five goals in a game, matching Luiz Adriano, who achieved the feat in 2014 for Shakhtar Donetsk, and Lionel Messi, in 2012 for Barcelona - with Guardiola as coach.
"I didn't know with Messi when it was happening against Leverkusen, they told me in the first interview," Guardiola said.
"But I made a substitution (Tuesday) because normally when the game is over, I want to...let them play, as many players as possible.
"Haaland equalled Messi's record, but he did it in 60 minutes, it's different. Who knows if he would have played 90 minutes?"
Haaland was a six-foot-four-inch bulldozer in front of the net all night, narrowly missing several other excellent chances in a victory that secured an 8-1 win on aggregate.
"I told (Guardiola) I would love to score a double hat-trick but what can I do?" said Haaland, who was hauled off minutes after his fifth goal.
Leipzig manager Marco Rose said he knew his team might be in for a stiff challenge against Haaland.
"He had a great night. He was so hungry," Rose said. "He scored goals with the foot and head, won second balls, deep runs. It looked really simple.
"He took the ball from the referee, that tells you it was a special night for him. Congratulations to Erling."
Haaland has 10 goals in the Champions League this season, and 33 in his career, reaching the 30-goal mark in 25 games, the fewest ever. He is also the youngest ever to reach that mark.
Guardiola has won nine major trophies at Manchester City but European success has eluded the Spaniard at the club. The closest they came to winning the Champions League was a 1-0 loss to Chelsea in the 2021 final.
