Lionel Messi handed Barcelona a fitness boost on Sunday as he returned to full training ahead of Tuesday's Champions League opener against Borussia Dortmund in Germany, a source has told ESPN FC.

The source also revealed that Messi took part in a mini-game with his teammates on Friday, too. The Argentine wasn't present for the 15 minutes which were open to the media, but did emerge afterwards as he stepped up his recovery from a calf injury.

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Messi, 32, injured his calf in his first training session of preseason on Aug. 5, ruling him out of the club's tour of the United States. Barca initially expected him to be out for three weeks but he has still not returned and has not featured in any of their four league games to date.

As Ernesto Valverde said on Friday, he's unlikely to be rushed back or risked against Dortmund, but he is in line to make his first appearance of the season against Granada next Saturday.

However, he could still travel to Germany this week -- although that doesn't mean he will play. Barca fly to Dortmund on Monday, with the squad expected to be announced by Valverde as close to their flight as possible.

Ousmane Dembele is also closing in on a return to action. The France forward did some light training with his teammates on Sunday but is still a week or so away from a first-team comeback.

Dembele injured his hamstring in the opening day defeat at Athletic Bilbao and was ruled out for five weeks.

Meanwhile, Luis Suarez made his return from a calf problem in Saturday's 5-2 win over Valencia, scoring twice as a second-half substitute.

With Messi, Dembele and Suarez all nearing full fitness, Valverde faces a potential selection headache down the line given how well their deputies have done while they've been injured.

Antoine Griezmann, Carles Perez and 16-year-old Ansu Fati have all had good games, with the latter stealing the show against Valencia.

He scored one and created another inside the first seven minutes as he made his first start for the club, prompting Valverde to say Barcelona must try and "control the hype" which is now surrounding the teenager.