Rumors about his health have circulated for the past month.
After announcing he has cancer, the 19-year-old Kessel said he is leaving the team Monday, and will undergo surgery to treat it.
Surgery was performed last Monday to remove the cancerous testicle. Tests Friday determined the cancer was confined to that spot.
"I was real nervous for the last week or so, but when I heard the prognosis (Friday), I was relieved by it," Kessel said on the team's Web site.
Boston's top pick in the 2006 NHL Entry Draft has played in 27 games. So far, he has five goals and four assists.
He has no goals and only one assist in the team's last 11 games.
Kessel is the second Boston athlete this year to be diagnosed with cancer. Red Sox pitcher Jon Lester was diagnosed with lymphoma in August.