The Paraguayan striker only moved to Vicarage Road in the summer, having joined from Girona.
Since then, the 25-year-old has scored just one goal in 12 appearances for the Hornets in all competitions.
That came in a Capital One Cup match against Norwich back in September.
Acuna now heads to La Liga in a bid to get more match time and regain his confidence.
At Girona last season, he netted 17 goals in the Spanish Second Division.