Fonseca, nicknamed el castor, started his football career at Nacional, his local team in Uruguay, in 1988. He played 14 games in two years and scored 3 goals. In 1990, he moved to Cagliari, scoring 17 goals in 50 appearances, playing mostly on the left rather than in his more habitual central position. In 1992 Napoli signed him and Fonseca managed a more impressive strike rate, scoring 31 goals in two seasons in Naples, including 5 goals in a 5–1 win against Valencia in the first knock-out round of the UEFA Cup on 16 September 1992. His form and performances drew attention from Roma, who promptly signed him in 1994. During his time with Napoli, he would occasionally put on the socks of the Uruguay national team, which is a shade lighter in colour than Napoli's socks, saying that they would bring him good luck. However, his three seasons were far from successful. Fonseca usually played as a second striker, supporting the Argentinecentre forwardAbel Balbo, but, because of the many injuries he suffered, he played discontinuously. Juventus signed Fonseca in 1997, but he was once again played out of position on the left wing, as he had been at Cagliari and Roma. His goalscoring record in Serie A was very good considering he was not always a first choice player with Roma and Juventus. Fonseca was always regarded as a "super sub", and scored several important goals coming off the bench. During his time with the Turin club, he won one Serie A title, a Champions League runner-up medal and the Supercoppa Italiana. Injuries ruled him completely out of the 1999–2000 season,, which saw him transferred to River Plate in Argentina. There, he infamously played only during the club's 2000 pre-season. His only match was a pre-season encounter against archrivals Boca Juniors, and Fonseca helped his club to earn a win by scoring the final penalty in the shootout. A few days later, Fonseca would resign from his contract and join Como in 2001. He retired in 2003 after the coach told him that he was no longer a part of the first team's plans.
Fonseca was a quick, opportunistic, and hard-working striker, with good dribbling skills, vision, and a powerful shot from distance. This made him capable of both scoring goals and creating chances for teammates. A versatile and well-rounded attacker, Fonseca was also effective in the air, and was capable of playing in several offensive positions. His pace, technique, defensive work-rate, and passing ability meant he could play on the left wing, as well as in a central role, or even in a more creative role as a second striker alongside or behind another striker. He was also an accurate penalty kick and set-piece taker. Despite his ability, however, he was also injury prone.
