Barca tactician Guardiola had revealed after his side's 4-0 victory that the club were preparing to announce that the Argentine forward would be sidelined for their first-leg clash at the Nou Camp after being struck down by flu and fever.
However, in somewhat surprising fashion, Messi was named among the substitutes in their last-16 match and promptly came off the bench in the second half to help himself to a double in Barcelona's eventual 4-0 win.
Osasuna gaffer Jose Luis Mendilibar hinted after his side's walloping that they were the victims of a hoax on Barca's part.
"I asked him [Messi] if he had a fever and had been sick as they had said and he burst out laughing," the coach revealed during his press conference after the game.
However, Messi downplayed the extent of his illness, only conceding that he was feeling slightly under the weather.
"[I] just had a tummy ache, nothing else," Messi told reporters afterwards.
"I never really had a fever. I felt good to come to the stadium and I tried to help the team."
His club colleague Sergio Busquets intimated that there was nothing surprising about Messi's astonishing recovery.
"His secret is that he is the best in the world," Busquets told Marca.
"Throughout the years, he has played almost every minute. In the morning, he was not feeling well but then he felt better and was able to play."
Barcelona are poised to face off with arch rivals Real Madrid in the quarter-finals after Jose Mourinho's charges stormed to a 3-2 first leg win over Malaga after going 2-0 behind at the Santiago Bernabeu on Tuesday night.