The 33-year-old was brought to London Stadium on a free transfer at the end of May as a back-up to established number one Lukasz Fabianski, signing a two-year deal.
But an early hip injury for Fabianski handed Roberto his opportunity in the first team.
The Spaniard played eight times in the league but was at fault for a number of goals during a slump in form and was eventually replaced by David Martin.
Fabianski suffered another injury earlier this month, yet West Ham have brought back Darren Randolph from Middlesbrough in a reported £4million transfer.
Roberto has subsequently been allowed to return to LaLiga on a temporary deal.