Rumors to that effect have been swirling since Daniels was hospitalized in June, just prior to the start of the OSU summer football camp.
At the time, it was thought that Daniels, 63, had a heart attack.
But Tressel said closer inspection showed that Daniels had cancer.
"We've got a plan, and Joe is getting back to work and getting into it," Tressel told Repository reporter Todd Porter. "He's back working half days and will probably only be able to work half days as he goes through treatment for some time."
According to the Repository report, Tressel was not specific about the nature of Daniels' cancer.
The coach noted that as part of the treatment for the apparent heart attack doctors put Daniels on blood thinning medication. That allowed them to detect two tumors that were bleeding.
Daniels has been on the OSU staff since Tressel's arrival in 2001. The veteran assistant coach is about to begin his 42nd year as a football coach. A native of Bethel Park, Pa., Daniels has made previous coaching stops at Boston College, West Virginia, Pittsburgh, Akron, Cincinnati and three NFL franchises (Cleveland, Buffalo and the New York Jets).
He has coached a who's who of football greats, including Dan Marino, Ken O'Brien, Andre Reed and Ozzie Newsome. He also helped develop Craig Krenzel and Troy Smith into Big Ten championship-caliber quarterbacks at Ohio State.
Daniels reportedly will work a light schedule when Ohio State opens fall camp Sunday as he undergoes treatment for the cancer.
Daniels was the second OSU assistant to undergo treatment for an apparent heart attack this off-season. Offensive coordinator Jim Bollman took ill during a recruiting trip to northeast Ohio. He returned to Columbus and underwent bypass surgery May 27 at OSU Medical Center.
Bollman made a brief appearance at the OSU football camp in June, although his status for preseason camp has not been announced.