The England international used to face the prolific Argentine during his time at Tottenham Hotspur, but now the pair are teammates following Walker's £50m summer move to City.
Aguero has scored 128 Premier League goals since joining the North-West outfit from Atletico Madrid in 2011, and 10 of those have been against Spurs.
The 29-year-old, who was linked with a move away from City in the summer, is just one goal away from equalling the club record of 177, which was set by Eric Brook.
"To play against, he's a nightmare," The Mirror quotes Walker as saying. "Facing him numerous seasons, he always seemed to score against us. It was frustrating. So having him on the same team is a big bonus.
"Listen, he's a quality player. You can see what he's done in the Premier League. Around the dressing room, it helps the younger lads. You can lean on him and he gives us advice.
"If things aren't going well, he is the kind of player who can produce that bit of magic and put us one goal up or back into the game."
Meanwhile, City are preparing to face Shakhtar Donetsk at home in the Champions League on Tuesday night.