A midfielder, Arnáez made his professional debut on 2 August 1987 for Puntarenas against Ramonense and he scored his first senior goal on 14 February 1988 against San Carlos.
[3] Nicknamed el Flaco (the Skinny one),[4] he made his debut for the national team in an April 1991 friendly match against Mexico, collecting a total of 31 caps and scoring 9 goals.
[9] Upon Alajuelense's triumph in the 2004 CONCACAF Champions' Cup, he was recalled to the national team at the age of 36 by coach Jorge Luis Pinto.
His final cap came in a 5–2 loss in World Cup qualifying against Honduras on 18 August 2004.
[12] He then had a spell as an assistant coach for the Costa Rica national football team[13] and was appointed manager of Herediano in December 2009[14] but returned to Puntarenas in March 2014.