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. 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. 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. After the season, the Indians engaged in talks with the Athletics to trade Rocco for Dick Siebert, who was unhappy in Philadelphia. The trade discussions eventually died down, however, and both players remained with their teams. Entering 1944, Rocco, who had spent the offseason working at a tavern and a war plant in St. Paul, was classified as 1-A by the Selective Service System, and was required to join the military to enlist in World War II. However, a stomach ailment caused him to be re-classified as 4-F and rejected for service, and he returned to the Indians for the 1944 Cleveland Indians season. He played in every game for the Indians that year, and ended up being inconsistent at the plate throughout the season; in July he had 13 at bats without a hit, but also had a 12-game hitting streak. Rocco led the league in putouts, assists, games played, and at bats, finishing the season with a .266 batting average, 13 home runs, and 70 RBIs.