Shandong are currently equal-second on points with FC Seoul on the Group F table but get to play the winless Sriwijaya in the final group match, prompting Tumbakovic to announce his confidence.
"We will definitely win the last group game to qualify," said Tumbakovic.
"Nothing can stop us from competing in the knockout phase.
"We actually expected to secure a place in the last 16 by beating Gamba today," he added.
"It was a crucial game for us and we were in full gear before playing the defending champions.
"But the game proved they were too good to beat and more importantly, they are experienced and they know how to play smartly."