"I never thought we were comfortable although the final result suggests we won easily," Mowbray said after the game.
"Ishmael Miller is a very powerful player while Roman Bednar, who's scored bags of goals for us this season, worked hard for very little reward.
"They had the opportunity to get back into after they scored to make it 2-1, but we scored the crucial third goal and from then on there was only one team that was going to win it.
"This weekend has been a great advert for the Championship, it's a very hard league to get out of.
"I've got no preferences as to who we should play in the semi-finals but it should be a great game at Wembley whoever we're against."