Ba took advantage of Gerrard's slip to tuck home the first in a 2-0 win over Liverpool.
Gerrard has been imperious for the Reds this season but Ba said: "I was waiting for a mistake, it wasn't a surprise.
"We knew we weren't going to get many chances.
"You have to keep going, you have to keep doing gambling and fortunately the mistake came.
"I didn't think about it, that's why I scored. If I'd thought about it I would have missed the chance."