The Blaugrana revealed on Saturday morning that the 30-year-old had withdrawn from the squad to face Malaga due to "personal reasons".
Messi announced in October 2017 that his wife, Antonella, was pregnant again, leading fans to suspect that the birth of his third child was the reason for his exclusion from the squad.
Indeed, the Argentine took to Instagram later on Saturday to introduce his newborn son Ciro to the world.
A post shared by Leo Messi (@leomessi) on
The social media post read: "Welcome Ciro! Thank God everything went perfect. Mom and he are very well. We are super happy!"
Messi is already the father to two-year-old Mateo and five-year-old Thiago.