Capel, whose side were a goal down with less than 20 minutes to go in the game, sealed Sporting's come-from-behind win after Emiliano Insua had tied the score at one goal apiece with 10 minutes remaining on the clock.
"The goal was a special moment," he said. "I am pleased.
"We did our work and it was a magnificent night. [The return leg] will be a complicated game, difficult.
"We must continue to do what we have done at this level."
Capel arrived at the Jose Alvalade from Sevilla last summer.