By Gabriel Tan in Bangkok
The War Elephants head into Saturday's game at the Suphachalasai Stadium trailing 3-1 on aggregate following Wednesday's defeat in Singapore.
However, they did grab a valuable away goal in the 59th minute through Adul, who was one of the few Thailand players who walked off the Jalan Besar Stadium pitch with his head held high after a trademark industrious performance.
Aware that his players were caught out by Singapore's aggressiveness, Schafer believes they need to show the same kind of spirit that Adul displayed if they are to overturn the deficit and win Southeast Asia's biggest prize for a record fourth time.
"The first match was not our match," he said in Tuesday's pre-match press conference. "We were better in the second half but the first half wasn't good.
"But I think we can change the 3-1 score. We'll have to change our play and concentrate on putting pressure on the opponents - I'm sure we can change the game.
"Singapore were very aggressive and were fighting in the last match but our goal that made it 1-1 - we have to play the same way tomorrow [Saturday] - it was heart and fight!
"It was fantastic from Adul. He fought to win the ball and saw what his team-mate was doing so he followed up to meet the second ball. We have to play with this kind of determination."
Thailand's display on Wednesday was also marred by injuries to a number of key players, including captain Panupong Wongsa, playmaker Datsakorn Thonglao and midfielders Anucha Kitpongsri and Adul.
Nonetheless, Schafer believes all of them will be fit enough to take to the field on Saturday, and maintained he has enough quality in reserve to match Singapore.
"I am sure everybody can play," the German added. "I don't know about losing - we only talk about winning.
"I cannot tell you which player will come in tomorrow [Saturday]. We have 22 players and depending on the situation I will see when a good time to make a change is.
"Now we have the inertia in Bangkok, I think we will change the match.
"My players are in good spirits and will be buoyed by playing in front of our own supporters. I hope the stadium will be full and we can show our goal in Singapore was very important."
Schafer is no stranger to success in an international tournament, having led Cameroon to the Africa Cup of Nations in 2002.