Dietmar Hamann told Sky Sports that Manchester City striker Sergio Aguero is "unplayable" when he is at his best.
The Argentine netted a brace at the Etihad Stadium on Tuesday evening as the Citizens thumped CSKA Moscow 5-2 to secure their spot in the round of 16 with two group matches remaining.
Former City midfielder Hamann saluted Aguero's attitude and electric pace, but also reserved praise for the 25-year-old's attacking ally, ex-Sevilla hitman Alvaro Negredo, who rounded off his hat-trick against the Russians in stoppage time.
"Aguero is unplayable at times," the German told Sky Sports.
"He works so hard, gets into good positions with his low centre of gravity, and when he is on the ball he can beat people with his dynamic bursts of speed.
"But he has a perfect strike partner in Negredo, who is a grafter, can hold the ball up, has a real physical presence, and someone who, if he gets his head on the ball in the six-yard box, you won't stop him from scoring."
Hamann's fellow Sky Sports pundit Ray Wilkins was also in awe of City's forward play as Manuel Pellegrini's charges racked up a quintet of goals, three days after they had blasted seven past Norwich in the Premier League.
But the onetime Manchester United engine-roomer fears the Etihad outfit - who will be joined in the last 16 by Group D rivals Bayern Munich - could be found out in the latter stages of Europe's premier club competition if they do not tighten up at the back.
Jamie Carragher described Gael Clichy's defending when he fouled Seydou Doumbia for CSKA's second-half penalty as "shocking" and Wilkins says City's rearguard must improve their attention levels for the tests ahead.
"They have one of the best attacking units on the planet," Wilkins said of City, who host pool minnows Plzen in their next Champions League encounter. "With the craft and guile they have, they are so good to watch going forward.
"But I'm sure Pellegrini will be slightly disappointed that on a couple of occasions they had lapses of concentration because to go forward in the Champions League and, perhaps, even win it, you can't have that.
"You have to be switched on from the minute the game starts to the minute the game ends."
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