The 21-year-old was told he was in the Albion team just minutes before kick-off after Jonas Olsson injured his groin in the warm-up.
Dawson was playing for non-league Radcliffe Borough three years ago but now feels at home at West Brom.
Dawson said: "Our assistant manager Keith Downing disappeared down the tunnel then told me 'Come on, get ready, you're starting.'
"It's the first time it's ever happened to me but you have to go into game mode straight away.
"There was no time to get nervous. Zamora made me work hard."