[1] Rocco began his professional career in 1935 with the Portsmouth Pirates of the Middle Atlantic League; he had a batting average of .340 in 95 games.

[7] Rocco made his major league debut the following day on June 5 against the Philadelphia Athletics, hitting a double and a triple in a 6–5 loss.

[8] Highlights from his rookie year included a three-double outing against the Washington Senators on July 8, as well as nine hits in 13 at bats in a late August three-game series against the Chicago White Sox.

[9][10] Rocco served as the everyday first baseman throughout the season, and finished 1943 with a .240 batting average and a league-leading .995 fielding percentage in 108 games.

[11] After the season, the Indians engaged in talks with the Athletics to trade Rocco for Dick Siebert, who was unhappy in Philadelphia.

It took until early April, just before the season started, for the two sides to come to an agreement; Rocco had spent the offseason training at the University of Minnesota and as a result did not miss any playing time despite the holdout.

[17][18] On September 9, 1945, Rocco had two singles, two doubles, and two home runs in a doubleheader sweep over New York in front of a crowd of 72,252 at Yankee Stadium, which he considered the highlight of his professional baseball career.

Fleming won the job and held it, though Rocco saw occasional playing time until June 26, when he was traded with cash to the Chicago Cubs for first baseman Heinz Becker in one of new owner Bill Veeck's first moves.

However, Rocco refused to report to the Nashville Volunteers, the Cubs' minor league organization, and considered quitting the game, which put the trade on hold.

[22] He finished the season with a .300 batting average, 27 home runs and 149 RBIs in 178 games, and was named to the PCL All-Star Team.

[24] Rocco won the job and held it for the year, finishing the season with a .258 batting average, 26 home runs, and 108 RBIs in 183 games.