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Denver Broncos linebacker Alex Singleton underwent testicular cancer surgery last week ... revealing a routine NFL drug test led to his diagnosis.

The 31-year-old posted a lengthy statement Monday morning ... starting it off by declaring, "This morning, I stood before our team as a lucky man." Getty

Singleton -- who had nine combined tackles against the Las Vegas Raiders on "Thursday Night Football" said he had a successful procedure a day later ... and the belief is the cancer was caught early.

The former CFL star went as far as to say he could be back on the field in a matter of weeks.

He detailed his journey ... saying, "A little over two weeks ago, following a random test as part of the NFL’s Drug Testing Program, I received a notice that I had elevated levels of the ho

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