Strop had entered the game in the eighth inning with the Cubs leading 2-1 against the Cardinals. He served up a game-tying leadoff home run to pinch-hitter Greg Garcia, then hit the next batter. Matt Carpenter then flied out but Strop walked Jhonny Peralta and was pulled.
As the right-hander walked off the field, he pointed to the sky, something Strop does after every outing.
"I always thank God for everything, for the opportunity," Strop said Sunday. "I always do that."
Costas offered his opinion during the MLB Network broadcast.
"Strop is on his way out, pointing toward the heavens," Costas said. "We can only ask, or wonder, that he is asking some departed relative for forgiveness for this atrocious performance."
Strop didn't know what Costas said until he got back to his hotel room and got reaction from fans and friends via Instagram.
"People said, 'Hey, don't worry about what Bob said,'" Strop said. "I was like, 'What's going on?' Then I went on Twitter and saw the comments that he said. I felt pretty bad. At the moment, really bad. I didn't want to talk to him. Somebody told me he wants to apologize to you, and I was like, 'I don't want to talk to him.'"
But Costas did meet with the pitcher in the lobby of the Cubs' team hotel on Sunday before Strop went to a fundraiser event with teammate Jason Motte.
"I saw him in the lobby and he apologized to me," Strop said. "We all make mistakes, and I'm not going to judge him because he made a mistake. I feel better now that he apologized. We all do stuff that we have to apologize for.
"I came down and realized he's a good dude," Strop said. "A lot of people told me he's a great guy and he's a legend and one of the best at what he does. Everybody makes mistakes."
Carrie Muskat is a reporter for MLB.com. She writes a blog, Muskat Ramblings, and you can follow her on Twitter @CarrieMuskat. This story was not subject to the approval of Major League Baseball or its clubs.