Despite concerns that a previous groin problem had resurfaced, Barcelona revealed it's the groin in his right leg this time, not his left leg.
Messi missed his country's recent World Cup qualifier against Venezuela -- having played with the injury against Uruguay -- after injuring his groin against Athletic Bilbao in a league game at the end of August.
After his return to Spain, he was rushed back into action against Alaves, coming on as a second-half substitute in Barcelona's 2-1 loss earlier this month.
Messi then played 90 minutes in comfortable wins over Celtic and Leganes, in the Champions League and La Liga, respectively.
Barca now face three away games, with trips to Sporting Gijon, Borussia Monchengladbach and Celta Vigo coming up before October's international break. Messi is expected to miss all of them.
He is also expected to miss Argentina's World Cup qualifiers against Peru and Paraguay on Oct. 6 and 11.
The Catalan side also lost Sergio Busquets to flu in the 50th minute of the draw with Atletico; the midfielder was replaced by Andre Gomes.
Enrique explained in his postgame news conference that Busquets had been struggling with illness in the buildup to the game and that by half-time he had no energy to continue.
