Welcome to Good Morning America’s worst nightmare: NBC sources tell the Daily News that Katie Couric may fill in as a substitute Today show co-host when Savannah Guthrie goes on maternity leave this summer. (If she doesn’t take over for Barbara Walters at The View.) Supposedly, Couric and Matt Lauer came up with the idea of a potential reunion when they ran into each other at a party last month, but there have been no official talks with Couric’s camp. It’s also possible that Meredith Vieira will fill in for Guthrie too. “With the Today show in a good place, we are blessed with great talent and many options to temporarily fill Savannah’s chair while she’s on leave, but there’s no plan yet,” an NBC News spokeswoman said. “Stay tuned.”