Monk brought on Hernandez after the break due to a hamstring injury to Marvin Emnes, and he said of his decision on Sky Sports 1: "It's something we were working on in the week, in my mind I had Pablo in there.
"He's got the quality and he can play wide as well. Marvin did well in the week and I thought with the dynamics and the pace (it) would give us a threat but Marvin was feeling his hamstring at half-time and I felt it was the right time to make the substitution.
"And I know the quality Pablo has and those are the kind of balls we were practising in the week. Obviously Wayne's latched onto one and put it in so it was fantastic."
The Swans boss knows what it is like to be involved in a south Wales derby and was pleased he could keep his head to give out instructions on the pitch.
He added: "I've played in a lot of these games and I know exactly what it means.
"It's about giving information and about getting the players to know what you want to get on the pitch.
"It's important I keep a clear head and get that information to them on the pitch."
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