On August 17, 2006, the Bruins announced that they had signed Kessel to a three-year, entry-level contract worth the rookie maximum of $850,000.

On December 11, 2006, Kessel's family announced that he was hospitalized for a reason unrelated to hockey, and WBZ-TV reported that Kessel was diagnosed with a form of testicular cancer.
On December 16, Kessel was pronounced cancer-free.
On January 5, 2007, he was assigned to the Providence Bruins, Boston's American Hockey League (AHL) affiliate, for conditioning purposes and then recalled on January 7.
Kessel returned to the Bruins lineup on January 9, against the Ottawa Senators, after missing only 11 regular season games following cancer surgery.

Kessel was named to the 2007 NHL YoungStars Game in Dallas on January 23, 2007.
He recorded a hat-trick (including a unique powerplay goal, plus the game-winner) and an assist during a 9–8 Eastern Conference victory.
While Kessel was not among top rookies in goals or assists, for the 2006–07 season he was second among rookies with four shootout goals (in seven attempts).
Each was a game-deciding goal.

At the conclusion of the season, Kessel was voted by Boston sports writers as the team's candidate for the Bill Masterton Memorial Trophy after battling testicular cancer.

At the 2007 NHL Awards Ceremony at the Elgin Theatre in Toronto, Kessel was officially selected as the recipient of the 2007 Masterton Trophy.

For the 2007–08 season Kessel led the league with 5 game-deciding shootout goals (out of 13 attempts in shootouts).
He set a Bruins career record with 9 shootout-deciding goals, surpassed by Patrice Bergeron seven years later.

Kessel scored the first Bruins goal of the 2008–09 season, in a 5–4 Bruins victory against the Colorado Avalanche.
Kessel ended the regular season on a high note, scoring his second career hat-trick in the April 12, 2009, 6–2 visitors' victory against the New York Islanders, and amassing the highest number of NHL regular season goals so far in his career with 36, the most on the Bruins that season.
He also tied Ed Olczyk for longest point streak by an American-born NHL player (18 games, 14 goals and 14 assists for 28 points during streak) in late 2008.
It is now the third-longest such point streak after Patrick Kane's 26-game streak in late 2015 and 20-game streak in the beginning of 2019.

Kessel played an integral role in Boston's run during the 2009 Stanley Cup playoffs, leading the Bruins with six goals before losing to the Carolina Hurricanes in the Eastern Conference Semifinals in seven games.
After the playoffs, it was reported that Kessel would need off-season shoulder surgery to repair an injury most likely incurred during a 2–0 loss to the Columbus Blue Jackets.
The surgery was successfully performed during the off-season, with recuperation forcing Kessel to miss the start of the 2009–10 season.
On August 17, 2006, the Bruins announced that they had signed Kessel to a three-year, entry-level contract worth the rookie maximum of $850,000.
On December 11, 2006, Kessel's family announced that he was hospitalized for a reason unrelated to hockey, and WBZ-TV reported that Kessel was diagnosed with a form of testicular cancer. On December 16, Kessel was pronounced cancer-free. On January 5, 2007, he was assigned to the Providence Bruins, Boston's American Hockey League (AHL) affiliate, for conditioning purposes and then recalled on January 7. Kessel returned to the Bruins lineup on January 9, against the Ottawa Senators, after missing only 11 regular season games following cancer surgery.
Kessel was named to the 2007 NHL YoungStars Game in Dallas on January 23, 2007. He recorded a hat-trick (including a unique powerplay goal, plus the game-winner) and an assist during a 9–8 Eastern Conference victory. While Kessel was not among top rookies in goals or assists, for the 2006–07 season he was second among rookies with four shootout goals (in seven attempts). Each was a game-deciding goal.
At the conclusion of the season, Kessel was voted by Boston sports writers as the team's candidate for the Bill Masterton Memorial Trophy after battling testicular cancer.
At the 2007 NHL Awards Ceremony at the Elgin Theatre in Toronto, Kessel was officially selected as the recipient of the 2007 Masterton Trophy.
For the 2007–08 season Kessel led the league with 5 game-deciding shootout goals (out of 13 attempts in shootouts). He set a Bruins career record with 9 shootout-deciding goals, surpassed by Patrice Bergeron seven years later.
Kessel scored the first Bruins goal of the 2008–09 season, in a 5–4 Bruins victory against the Colorado Avalanche. Kessel ended the regular season on a high note, scoring his second career hat-trick in the April 12, 2009, 6–2 visitors' victory against the New York Islanders, and amassing the highest number of NHL regular season goals so far in his career with 36, the most on the Bruins that season. He also tied Ed Olczyk for longest point streak by an American-born NHL player (18 games, 14 goals and 14 assists for 28 points during streak) in late 2008. It is now the third-longest such point streak after Patrick Kane's 26-game streak in late 2015 and 20-game streak in the beginning of 2019.
Kessel played an integral role in Boston's run during the 2009 Stanley Cup playoffs, leading the Bruins with six goals before losing to the Carolina Hurricanes in the Eastern Conference Semifinals in seven games. After the playoffs, it was reported that Kessel would need off-season shoulder surgery to repair an injury most likely incurred during a 2–0 loss to the Columbus Blue Jackets. The surgery was successfully performed during the off-season, with recuperation forcing Kessel to miss the start of the 2009–10 season.
Phil Kessel