With the scoreline at 2-1, City substitute Edin Dzeko got on the end of a James Milner cross but Boyce threw himself at the ball and managed to divert the Bosnian's effort over the crossbar in a heroic display to keep his side ahead.
Wigan went on to win at the Etihad Stadium to move into the semi-finals, where they will meet Arsenal, and Rosler was thrilled for veteran Boyce as well as goalscorer James Perch who added to Jordi Gomez's first-half penalty to set up the win.
"I just saw the block - unbelievable," said Rosler.
"I'm a big believer that you need match-winners at both ends, not only in the attacking box, but in your own box. That tackle was a match-winning, decisive moment for us.
"Both tackles, the one Perchy had when he scored, and the one Emmerson Boyce had when he denied Dzeko the goal, that has shown the desire and the will-power we have shown today. I can only take my hat off to the players.
"Emmerson is a symbol of that, not only today but since I've worked in the club."
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