Despite a remarkable year which has left Messi four goals shy of equalling Gerd Muller's all-time single-season scoring record of 67, questions have been raised about the South American's failure to find the net in recent matches.
Messi drew a blank against Chelsea in last week's 1-0 first leg defeat at Stamford Bridge and cut a frustrated figure once more at the weekend as the Catalans suffered a 2-1 loss to Real Madrid at Camp Nou.
News that Messi had missed training on Sunday also triggered alarm about his fitness for Tuesday's return match.
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However Guardiola told a press conference on Tuesday the FIFA world player of the year was fit to play.
"Leo had a gastric issue, the doctor saw him and he called us before the training session to say he wouldn't be coming in, but now he's okay," Guardiola said, before waving off questions about Messi's mini goal-drought.
"Allow me never to worry about Lionel Messi," Guardiola said.
"He cannot score for 27 games but then he will score three goals or give three assists. So let's give him licence to spend however long he needs without scoring.
"How many goals has he scored this season? He's scored an incredible amount. He lives for scoring, helping his people. Let him be happy on the pitch."
Guardiola meanwhile said he was confident that if Barcelona were able to reproduce their performance of the first leg -- when they enjoyed 70 percent possession, hit the woodwork twice and carved out a string of clear chances -- they would be able to overturn Chelsea's 1-0 lead.
"We will have to have balance between rhythm, pace, patience and intensity," Guardiola said. "We need to think that we have 90 minutes ahead of us and that with patience and intensity we can make it."
He also dismissed concerns that Barcelona would struggle to raise themselves for what their third major match inside six days.
"Tomorrow there's no tiredness in the legs. Maybe in the mind. But in the semi-final of a Champions League, no matter if we play three days ago or yesterday -- tomorrow we'll be ready to try and reach the final."
