The 31-year-old - without a club since 2013 - scored the goal that put Iraq 2-1 up in extra-time, but the sides ended the additional 30 minutes tied at 3-3 following a frantic period of play.
Mahmoud's Panenka helped Iraq to a 7-6 win in the shootout, and Shenaishil was quick to recognise his skipper's contribution.
"Younis essentially played four halves today and he played amazing," he said. "We have a young squad and we need a leader. He's the type of player that opponents hate to play against and team-mates love to play with.
"There was some doubt in the media about whether he should play or not, but I don't listen to them. As a coach I see what the player produces during training time and on the field.
"I wish him all the best. He's a star and important for Iraq."
Shenaishil believes a red card for Iran defender Mehdrad Pooladi when Iraq were a goal down in the first half proved to be a turning point.
Pooladi initially looked to have escaped a red card after receiving a second yellow, but referee Ben Williams was reminded of the earlier booking by a host of Iraqi players.
"I think the red card maybe meant their players lost some concentration," he continued. "They were leading 1-0 and they should have been more relaxed.
"I have all respect for the Iran team but they had too much pressure on them as players in the match."
Iraq will now meet South Korea in the semi-finals, and Shenaishil knows his side must recover quickly ahead of Monday's fixture.
"Our main goal now is recovery," he added. "We played four halves in this game and the main thing now is recovery ahead of the semi-final."
