Two years after being drafted by Boston out of high school, Rico Petrocelli made his Red Sox debut at Fenway Park in late September 1963, hitting an RBI double in his first at-bat in his only appearance that season. Six foot tall and just a shade under 200 pounds, Boston recognized that he would add some pop to the lineup once he was ready to be an everyday major league ballplayer, so the Brooklyn native spent 1964 playing for Triple-A Seattle before he finally made the team for good in the spring of 1965. In his rookie season, he managed just a .232 average on a team but also clocked 13 home runs and knocked in 33 runs on a club that hit rock bottom with 100 games lost. He also showed solid play at short; even with 18 errors and a .958 fielding percentage, the unusual combination of power and glove work normally uncommon among infielders made him a valuable commodity for the franchise.

In 1966, the second-year player hit another 18 home runs, including two grand slams, and drove home 59 base runners; the following season, as his team with 100-to-1 odds to win the pennant shocked fans and baseball alike, Petrocelli batted a respectable .259 average while hitting 17 home runs, driving in 66, and making the first of two All-Star appearances. He also managed a fielding percentage of .970 in 141 appearances while also assisting in 73 double plays; he even made the final out of the season, catching a soft pop-up in shallow left that clinched the pennant for Boston. His efforts were enough to earn consideration for the American League Most Valuable Player, though he finished well behind teammate Carl Yastrzemski for that honor.

Petrocelli continued to flex his impressive power at the plate; in 1969, he batted a career-high .297 while amassing 40 home runs, then a record for a shortstop, and drove in 97 runs while crossing the plate 92 times. He also improved defensively, too, committing just 14 errors, which tied a then-record at that position, and assisting in 103 double plays. Over the next two seasons, he would collect another 57 home runs and 192 RBI and also began the transition from shortstop to third base; when the Red Sox signed future Hall of Fame player Luis Aparicio prior to the 1971 season, Petrocelli became the full-time third baseman in Boston and responded by making just 11 errors and leading the league in fielding percentage at his new position.

Though he continued to perform well in the field, his offensive numbers took a sudden downward turn after that season. In 1972, his home run total fell from 28 the year before to just 15, and his batting average was back down to just .240. Then, injuries began to take him away from the game; elbow problems in 1974 caused him to miss time at the start as well as the end of that season, though he still managed 13 home runs and 76 RBI. The following year, Petrocelli began the season hitting only .136 through the first month of the season and didn’t stroke his first home run until the middle of May; a leg injury halfway through August then put him on the disabled list for better than two weeks and finished the season with just seven home runs and 59 RBI to his credit, totals well below his career average. He would play one last season with Boston in 1976 and, upon his release before the end of spring training in 1977, finish his 13-year career with 210 home runs, 773 RBI, and a .251 batting average along with the recognition as one of the best power-hitting infielders in Red Sox history.