Inserted into the rotation, he won his first three decisions for the Yankees before losing a game on June 7. However, on June 12, he was placed on the 21-day disabled list with a strained left Achilles tendon; the injury kept him out of action until August 8. On August 24, rookie Mark McGwire of the Athletics had two hits off him. McGwire's father happened to be John's dentist, and John told reporters, "When your dentist's kid starts hitting you, it's time to retire!" He went on to pitch three more seasons, however. Against the Mariners in the first game of a doubleheader on August 30, he took the loss but allowed just one run in eight innings in what would be his last start of the year. On September 4, he fell down while warming up on muddy soil and injured his left thumb while trying to break his fall, ending his season. In 13 games (10 starts), he had a 5–3 record, a 2.93 ERA, 28 strikeouts, 15 walks, and 73 hits allowed in innings pitched. He also made three starts for the Class A Fort Lauderdale Yankees, winning two and not allowing an earned run in innings. During the 1986 season, John had agreed to become the pitching coach for the University of North Carolina (UNC) later in the year, meaning he would likely retire. However, he resigned from the UNC position on November 21 due to "irreconcilable differences" with head coach Mike Roberts. In his 1991 autobiography, John stated that the reasons he resigned were that Roberts, after promising John two cars and six season tickets to UNC basketball games for coming to work for UNC, tried to cut the deal back to one car and no tickets. Roberts also used John's name on promotional materials for baseball camps without John's permission. After resigning from UNC, John signed a contract for $350,000, provided he made the team out of spring training. On April 19 and April 26, 1987, he threw seven shutout innings in consecutive starts, only allowing one hit on the 26th in a 14–2 win over Cleveland. Pitching on three days' rest on August 8, he threw a two-hit shutout against the Tigers, the last he would throw in his career. On October 2, he gave up just one run in a complete game, 3–1 triumph over Baltimore. In 33 starts, John had a