Mari joined the Gunners from Brazilian side Flamengo on an initial six-month loan deal in January for a fee of around £4milllion.
The defender made his debut for the U23s in defeat to Chelsea on Monday.
"Great feelings during my first minutes in Arsenal colours yesterday!" Mari wrote on Twitter. "Keep working hard!"
Great feelings during my first minutes in @Arsenal colours yesterday! Keep working hard!/ Buenas sensaciones en los primeros minutos con estos colores! Seguimos trabajando!/ Boas sensações durante meus primeiros minutos com essas cores! Continue trabalhando! pic.twitter.com/MSKGPRqVZA — Pablo Mari Villar (@PabloMV5) February 18, 2020