Grau made his senior debut on 21 August 2016, playing the last 14 minutes in a 2–0 home win against SD Amorebieta. The following July, after contributing with 25 appearances during the campaign, he returned to Los Blancos and was assigned to the reserves also in the third division; on 20 October 2017, he extended his contract until 2019.[3]
On 24 August 2020, Grau moved abroad with Primeira Liga side C.D. Tondela, after agreeing to a one-year loan deal.[6] Upon returning, he made his debut in the Spanish top tier on 18 September 2021, replacing Darko Brašanac in a 2–0 away win over Deportivo Alavés.[7]
On 19 January 2022, Grau signed a three-and-a-half-year contract with Real Zaragoza in the second division.[8] On 22 August, he terminated his link with the club.[9]