Wanamaker saw his role expand significantly in his second NBA season. The 30-year-old appeared in 71 of the Celtics' 72 games, drawing one start and seeing his average minutes rise from 9.5 as a rookie to 19.3 in 2019-20. Along the way, Wanamaker posted 6.9 points, 2.0 rebounds, 2.5 assists and 0.9 steals per game -- all career bests. Most remarkably, though, he led the entire NBA in free-throw percentage with a .926 mark, missing just 10 attempts from the charity stripe all season. Wanamaker chipped in double-digit points off the bench on 18 occasions. On Dec. 22 in a home win over the Hornets, Wanamaker dished a career-best eight assists. Arguably his finest effort came Feb. 3, when he delivered a season-high 16 points along with five assists, three rebounds and a career-high four steals against Atlanta. After the coronavirus pandemic put the season on pause, Wanamaker was even more effective, shooting 59.1 percent from the field en route to 9.3 points per game in seeding play. He remained a regular member of the Celtics' rotation throughout their run to the Eastern Conference Finals, averaging 4.9 points with 2.0 rebounds, 1.8 assists and 0.7 steals per game. In Game 5 of Boston's second-round series against Toronto, Wanamaker posted a career-playoff-high 15 points, three rebounds, two assists and steal in the win.

After seven years of proving himself overseas, Wanamaker finally became an NBA rookie during his age-29 season in 2018-19. Wanamaker's body of work in France, Italy, Germany and Turkey convinced the Celtics to sign him to a one-year contract in the summer of 2018. The University of Pittsburgh product ultimately saw action in 36 games with Boston as well as two matches with the G League's Maine Red Claws. With the Celtics, the point guard averaged 3.9 points, 1.1 rebounds and 1.6 assists over 9.5 minutes per game. On Jan. 14, with Kyrie Irving out, Wanamaker played 26 minutes, producing 13 points, four assists, three rebounds and two steals against the Nets. Nine days later, Wanamaker delivered 11 points in a home win over the Cavaliers. Then, during the last game of the season, Wanamaker scored a career-high 17 points to go with seven dimes and six boards in a win over the Wizards. Wanamaker went on to appear in four of the Celtics' nine playoff games. In Game 2 of the second round against Milwaukee, he scored seven points with a steal and an assist in just 7:04 off the bench.