On 2 January 2019, he was loaned to Boca Juniors. On 2 May 2019, Alonso was an unused substitute as Boca defeated Rosario Central in the 2018 Supercopa Argentina on penalty-kicks after the match ended 0–0.
On 29 May 2019, Alonso was selected to the final 23-man roster for the 2019 Copa América in Brazil. He would go on to start in all four of Paraguay's matches, playing every single minute, as they lost in the quarter-finals in a penalty shoot-out to hosts and eventual champions Brazil.