The England winger came off the bench to score Arsenal's first goal in their unlikely first-leg comeback, when they came back from two goals behind to draw 2-2 at the Emirates stadium.
Walcott's pace seem to worry the Catalans' defence, but Guardiola insisted the real reason Barca failed to win was down to Arsenal goalkeeper Manuel Almunia, who made a succession of fine saves to deny the Catalans, particularly in the first half.
"Let's be clear, Walcott was not the determining factor, the determining factor was Almunia, who saved 12,000 balls," the 39-year-old said.
Barca left-back Maxwell was criticised in the Spanish press for failing to cope with the pace of Walcott as Arsenal got back into the game.
"He is fast," the former Ajax defender said after the game.
"I looked to the linesman to see if the flag was up but by the time I looked up and looked back, Walcott was gone."
Guardiola defended the Brazilian on Friday, but the Barca coach is set to deploy fit-again Eric Abidal on the left after the France defender made an impressive return from injury against Athletic Bilbao at the weekend.
Abidal is much quicker than Maxwell and the former Lyon full-back is clearly unimpressed by the Englishman's ability to run 100 metres in just over 10 seconds, a feat highlighted by the local media in Monday's press conference, in which Walcott appeared alongside his coach, Arsene Wenger.
"It may be really good for him to run 100 metres in a little over 10 seconds, but the aim (in football) is to get to the goal," Abidal said on Saturday.
"We'll wait there for him and we'll see what he can do with the ball - let's see if he can get past four defenders."
With Abidal almost certain to replace Maxwell, three of the four defenders will be different to the ones that started in north London on Wednesday.
Carles Puyol and Gerard Pique, currently the club's two first-choice centre-backs, both miss out through suspension, with the latter also injured.
With summer signing Dmitro Chygrynskiy ineligible, Gabi Milito and Rafa Marquez look set to start tomorrow.
Milito is still feeling his way back following more than a year and a half on the sidelines, while Marquez has been plagued by injury and poor form this season.
Walcott may quietly be relishing the chance to run at those two, but Guardiola says keeping the ball, something Barcelona do very well, is the key to denying the England winger and his Arsenal team-mates at the Nou Camp.
"Walcott is so speedy, he has strong legs, he played very well against us (on Wednesday) and also in the last league game," he said.
"But we will try to keep the ball, that's the best way to stop Arsenal and to stop Walcott as well."
And the Barca coach says his side will go out and play their usual attacking game.
"We are going to try to be ourselves, to keep the ball, to get the ball and play with great intensity," he said.
"And I think Mr Wenger will want to do the same."
Barcelona could only draw in London, despite enjoying almost 70 per cent of the possession on the night.
But Guardiola says the side that has the ball most at the Nou Camp will be the one that progresses to the semi-finals to face either Inter Milan or CSKA Moscow.
"The team that has the ball most will win the game," he said.
"Let's hope that's us, we have to keep going - tomorrow is our biggest game of the season."