Messi was forced off after less than half-an-hour of the match on Sunday after pulling up with a muscle problem.
He now looks set to be ruled out of Argentina's forthcoming friendlies with Ecuador and Bosnia and Herzegovina because of the problem.
Barcelona confirmed via their Twitter feed that Messi will undergo tests further tests on Monday.
Barca are also due to check on Cesc Fabregas - who like Messi - was taken off against Betis.
Fabregas was replaced, after he scored twice in the 4-1 win, with a knee problem.
After the game Fabregas said: "I had already felt something against Milan.
"I started well today, but felt something again so I preferred to stop. I had an injury there three years ago."
On team-mate Messi he said: "I had the same as Messi at Arsenal, I had problems with injuries over one-and-a-half years.
"Now I think he should make sure he recovers totally."