From Gerardo Martino, via Dermot Corrigan of ESPN FC:

[Messi] looked fine to me. He got the equaliser for us. He played with speed, intensity, looking for one-on-ones.

With Cesc [Fabregas] on the pitch, he played on the wing and did it well. I believe there is now no doubt about his physical condition. He is over his injury.

From Dermot Corrigan of ESPNFC:

Gerardo Martino says Lionel Messi should be fine to start Barcelona's Champions League group game at AC Milan on Tuesday night:

“We did not want to risk him from the start (against Osasuna). He recovered much more quickly than we thought -- it was expected he would only be back in training this week -- so we thought it most prudent to give him just some minutes in Pamplona so he would feel better ahead of the games coming up now. He has the medical all-clear, so the risks are the same as with any other player.”

From Barcelona's official Twitter account:

Official: Puyol, Mascherano, Cuenca and Messi get the medical green light #fcblive (via @barcastuff)

— FCBarcelona (@BarcaStat) October 18, 2013

UPDATE: Friday, Oct. 18

From Lionel Messi's official Instagram account:

"Ready to come back."

UPDATE: Monday, Oct. 14

Barcelona have confirmed on their team website that Lionel Messi has returned to full training for the first time since he first suffered his thigh injury.

UPDATE: Tuesday, Oct. 8

From ESPNFC:

(Gerardo) Martino told a public meeting in Barcelona there was a chance that Messi, along with teammates Jordi Alba and Javier Mascherano, could return at Osasuna on October 19, meaning all three should be set for the following weekend's Clasico against Real Madrid at the Camp Nou.

"I do not know if he will be back for the game against Osasuna," Martino said. "From there, we can see about the next ones. But Jordi Alba and Mascherano will be there [at Osasuna]."

UPDATE: Monday, Oct. 7

Lionel Messi posted a photo on Instagram during treatment:

UPDATE: Sunday, Sept. 29

Barcelona officially declared that Lionel Messi will be sidelined for at least two weeks.

Tests carried out by the Club’s medical services this morning have confirmed the initial diagnosis that Leo Messi has suffered a slight thigh injury in his right leg.  The Argentine will be out for between two and three weeks, which means that he definitely won’t be available for the two matches coming up for FC Barcelona this week: Tuesday in Glasgow against Celtic, and Saturday in the Camp Nou against Valladolid.  That will be followed by another international break meaning that Barça will have to wait until 19^thOctober to resume the league programme against Osasuna in Pamplona.

UPDATE: Saturday, Sept. 28

From Barcelona's official Twitter feed: 

Messi sustained a right thigh muscle injury in the first half of the match against Almería. He will undergo additional tests on Sunday.

— FC Barcelona (@FCBarcelona) September 28, 2013

Messi made sure to make his 27-plus minutes count, putting Barca up, 1-0, with a goal in the 21st minute. Even without him, you'd expect the Spanish giants to be able to maintain that lead and walk away with the full three points.

FOX Soccer Trax speculated that it could be an old injury creating problems for Messi, rather than something new that happened during the match.

28 Xavi has come on for Messi at Almeria; Messi not moving well after scoring; maybe that old leg injury flaring again

— FOX Soccer Trax (@FOXSoccerTrax) September 28, 2013

The 26-year-old suffered a thigh injury toward the end of August in Barcelona's first-leg match with Atletico Madrid in the Spanish Super Cup. Toward the end of last season, Messi also looked to be less than fully fit as Barcelona were bounced out of the Champions League by Bayern Munich.

The big question is what this will mean for Barcelona going forward.

Kieran Canning of the AFP thinks Messi is unlikely to be on the pitch when the club travels to Celtic Park on Tuesday.

Messi forced off with that looks like another injury problem, must be a big doubt for Celtic game in midweek now

— Kieran Canning (@KieranCanning) September 28, 2013

It could be a tricky match for Barcelona, as the reigning Scottish champions beat Barca, 2-1, in Glasgow last season, and played very well against AC Milan in their first match of the group stage.

The signing of Neymar was supposed to help take some pressure of Messi, though. With the Brazilian forward in the side, Gerardo Martino shouldn't have to worry as much about how his attack is going to be negatively affected without its best player on the pitch.

Check back for updates on Messi's health when they become available.