Keith Hernandez’s cool been-there, done-that vibe long has been part of his charm. But later this week he will begin a new assignment that he admitted has him “really pumped.”

“I just pinch myself,” he said. “I’m tickled to death.”

Fox announced Monday that Hernandez and David Ortiz will join its postseason studio cast with host Kevin Burkhardt and analysts Alex Rodriguez and Frank Thomas, replacing Pete Rose, with whom Fox parted ways in August.

Hernandez is an iconic former player for the Cardinals and Mets and has been an SNY analyst since 2006, but other than a brief stint on games for The Baseball Network in the mid-1990s, this will be his debut as a national analyst.

“I’m thrilled to death that I’m involved in this,” he said. “I understand that this is a big deal. It’s national TV . . . But it’s not like I haven’t done something like this before. I know the game and I’ve watched the show and it’s conversational and talking about the game.”

Fox’s baseball studio show earned strongly positive reviews the past two seasons, with one key being the byplay between the straight-laced Rodriguez and off-beat Rose.

But Rose lost his job in the wake of decades-old allegations of sexual relations with a minor, forcing Fox to look for new personalities big enough to make a splash. Enter Hernandez and Ortiz, who retired from the Red Sox after last season.

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After Hernandez’s name came up as a possibility, he had lunch in Manhattan last month with John Entz, Fox Sports’ president of production, and producer Bardia Shah-Rais, and soon thereafter he was invited to come aboard.

Hernandez expects a smooth transition, because he worked closely for eight years at SNY with Burkhardt, and he also knows Rodriguez — a longtime fan of Hernandez’s.

Still, the group will not have the benefit of any regular-season practice reps. “It’s a whole new team, and we’re going to be thrown into the water together,” Hernandez said.

Hernandez, 63, now is guaranteed to return to the World Series, in a similar role to the one he played for SNY in the Mets’ 2015 postseason run, only this time on a larger stage.

Because Fox has the American League playoffs and Hernandez spends most of the season watching only National League teams, he has been working to “buff up” on the AL playoff entries.

“I just figure certainly I’m qualified for this; I know the game,” he said. “But this is a higher level right here. I’m just thrilled that they even considered me.”