The Waratahs were comprehensively beaten 47-17 in the penultimate warm-up game for the Lions ahead of next week's opening test against the Wallabies in Brisbane and the dominance of the Lions forwards played a key role in that defeat.
"I think it's very clear that the Lions are excellent at the ruck, both sides of the ball attacking ruck and defensive ruck," Cheika told a news conference.
"We were putting sledge hammers in there trying to get to the ball to pick it up.
"I thought we had a couple of steals but unfortunately they weren't given... I think the ruck will decide the flow of the game against the Wallabies.
Cheika was familiar with many of the Lions players from his time at Irish club Leinster, who he led to a Heineken Cup triumph before returning to Australia.
"I know they're class players. When they're all together like that the four nations they're pretty hard to stop," he added.
"You've got to put heat on the forwards. You've got to keep them hopping so you give yourself some chance of shutting down their attack."
Turning to the Waratahs' performance, Cheika described the game as a good physical battle but said his team had maybe not had "enough X-factor" to finish off some opportunities.
Waratahs' captain and number eight Dave Dennis, released from the Wallabies training squad for the match, said the power of the Lions rucking would be one message he would take back to Australia coach Robbie Deans.
"The emphasis on the ruck is strong and they commit a lot of guys in there," he said. "There were times when I felt they weren't supporting their weight they were pilfering the ball but they do it well. They put a lot of pressure on our ball and they got away with it."