A 6'5" (1.96 m) forward Guarilia played for the George Washington University varsity basketball team beginning in the 1956–1957 season.

In December 1959 Boston placed Guarilia on its farm out list to trim their roster to the required 10 player limit.

Red Auerbach considered rotating Guarilia on the active list with one of the other Celtics rookies, like John Richter.

Guarilia was particularly effective in Boston's 1961–62 NBA championship victory over the Los Angeles Lakers.

On September 25, 1963, the Celtics placed Guarilia on waivers to reduce their playing squad to 16 players.