Brett Favre says he was floored when Deion Sanders revealed this week he battled bladder cancer … but the legendary quarterback tells TMZ Sports he’s grateful Coach Prime’s “prognosis looks good.”

At a press conference on Monday, Sanders announced he had his bladder removed after doctors found a malignant tumor on it. Fortunately, his medical team said they caught everything in time, and the Colorado head coach is now cancer-free.

Favre — who played with Sanders in the NFL for one season in Atlanta — said the whole ordeal “totally shocked” him, though he couldn’t have been happier to hear his old friend is going to be OK.

“Thank God they found it and it was a successful surgery,” Favre said. “But I was completely caught off guard with the news but I’m thankful that the prognosis looks good.”

Favre told us he’s sure things will stay that way, too … because he said Sanders’ work ethic is second to none.

In fact, he told us back when the guys were Falcons, Deion “played every play in practice as if it were the Super Bowl-winning play.”

“So,” he said, “I’m not surprised he’s digging his heels in and is going to keep coaching.”

Favre was recently diagnosed with his own serious medical ailment — Parkinson’s — and when asked if he had any advice for Deion going forward, he kept it simple.