The Argentine attacker was a surprise absentee as the Blaugrana beat the Chinese champions 3-0 on Thursday as the Catalan club announced he was suffering from the kidney problem.
Messi has still not trained in Japan, but is by no means ruled out of Sunday's final against River and, according to medical sources contacted by Goal, the pain should not stop the four-time Ballon d'Or winner from starting in the final - providing he is feeling strong ahead of that match.
Those same sources said that there will be no risk of injury for Messi, who recently spent two months on the sidelines with a knee problem, even if he starts Sunday's game.
The 28-year-old trained normally on Wednesday and has only missed one session so far in Japan, while the club revealed he had awoken with less pain on Friday.
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Neymar, meanwhile, has still not trained fully, although he did take part in ball work and speed drills on Friday, and has now been out for nine days after picking up a muscle injury ahead of the Champions League clash against Bayer Leverkusen.
Unlike Messi, the Brazilian will need at least one full session to see if he is ready to face River and Barca hope he will be able to participate in training on Saturday. If he is less than 100 per cent, however, he will be unlikely to feature in the starting XI the following day.
"Nobody knows if they can play or not," Barca boss Luis Enrique said on Thursday. "Not even they do. We won't know until Sunday morning."