Jordan Ayew, 26, came on as a second-half substitute against Championship side Sheffield Wednesday at the Liberty Stadium to open the scoring in the 55th minute, before Nathan Dyer's late strike secured a 2-0 win in the fifth round replay.
The Swans are now unbeaten in their last 10 games in which the Ghana international has scored and with his man of the match display, Carvahal was left pleased by the eccentric forward's match-winning contribution.
"At the break, we weren't happy with the score. We needed more energy and intensity, so we brought on Martin Olsson and Jordan Ayew and they had a big impact," he said, as quoted via BBC Football. "Two or three minutes into the second half I thought things would change."
Swansea will now face either Tottenham or Rochdale in the last-eight of England's oldest cup competition.