The 22-year-old Brazil striker left the home defence for dead and stunned a packed Nou Camp to score the fifth-fastest goal in the Champions League since Europe's elite club competition was created in 1992.
"Not even Usain Bolt would have been able to stop him," Guardiola told a news conference. "I'd like my team to have legs with so much running in them."
Holders Barca fought back through strikes from Pedro and David Villa in the Group H opener before Thiago Silva snatched a stoppage-time equaliser.
"Certain things were against us, but all we can do is congratulate Milan and Pato."
With captain Carles Puyol only declared fit this week after knee surgery and his partner in central defence Gerard Pique sidelined, Barca coach Pep Guardiola had deployed midfielders Sergio Busquets and Javier Mascherano at the back.
The decision looked to have backfired immediately when Pato exposed their obvious lack of pace.
Former Netherlands striker Roy Makaay holds the record for the fastest Champions League goal after he netted for Bayern Munich 10 seconds into their game against Real Madrid in 2007.
Guardiola said there were some things he and his players needed to improve upon but said he had complete faith in the squad, which was reinforced in the close season with Spain midfielder Cesc Fabregas and Chile forward Alexis Sanchez.
"You all know already that I cannot doubt these players," former Barca and Spain midfielder Guardiola said.
"If they have demonstrated anything it's that in terms of work rate nobody comes near them."
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