The Chinese champions are billed as one of the favourites for the Asian crown and recorded a fine 3-1 win over Kashiwa Reysol on Tuesday
The Chinese champions lead the group with seven points ahead of Buriram United and Jeonbuk Motors who are both on six points, with Kashiwa a further three back.
Despite the victory over Reysol, Lee insisted the group was far from decided.
"This was the fourth match, and we still have two to go. The result in this group will not be decided until the last match and we must be serious about the upcoming games," he said.
However, Lee was content with his side's performance in the victory over the Japanese champions, where two second-half Muriqui goals settled the contest after Hiroki Sakai's 55th-minute equaliser.
"In the away game in Japan we did a better job in terms of ball control and also in other aspects, but because of Kashiwa's marking we failed to score," Lee said.
"Today in the second half we made some changes and created open space and we took advantage of this. If we had not worked harder after the equaliser, the result would have been different."
Meanwhile, Reysol boss Nelsinho Baptista said: "We played at 90 per cent for the whole game. We did a good job in the first half, but two mistakes meant our efforts were in vain."
Follow Ben Somerford on