Watson - who nabbed the unlikely late winner against Manchester City in last year's final - fired a superb first-half strike from just under 30 yards.
"I didn't saw (see) the second goal," Rosler said. "I just turned around and the ball was in."
Rosler added: "We take it game by game. I felt very happy with the performance, the application.
"I thought our supporters, they travelled in big numbers. We made them proud, we wanted to make them proud.
"I wanted to compete against the best teams in the country and Cardiff today gave us the possibility, and we stood our own ground very well.
"We've beaten now two teams (Premier League) on the way and we're now in the last eight."
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