Aguero needs to work harder, it's simple. He shouldn't be good only when others put in the hard yards. He needs to be constantly moving. One thing I've seen from Sane, Sterling and Jesus is that they give the ball away quite often but chase and chase to win it back. That's the difference from the likes of Aguero (at times) and Dzeko who would give it away and then give up on the ball.
You don't mind your players trying flicks or tough passes and giving the ball away if you see them hunt for it back. Sterling will gain so much confidence in the current side because usually a misplaced pass leads to a groan from the crowd, now we'll see him hunt for the ball back and be impressed. Jesus and Sane give it away fairly regularly but that fight they show wins the crowd over.If Aguero steps up and shows the same energy he'll start. He didn't when he came on though.
Aguero is trying hard, I'm not entirely sure why people have started thinking he isn't - perhaps it's because they'd rather he just needed to try harder instead of actually adapt his game which is much harder (and less likely) for him to do.
He's running over 50% further than under Pellegrini, and he's now running as much as everyone else in the team, as was covered in loads of newspapers around November.
Not only is he trying hard, but his pressing has become much more effective, and he's now doing some very effective and quite complex pressing movements detailed in the Spurs match report here.
http://spielverlagerung.com/2017/01/24/city-dominate-with-intense-pressing/
The fact is, Aguero is doing a lot of what Pep wants him to do - that's not the problem anymore. The problem is that he's in very poor form in front of goal, and it seems he can either play Pep's way, or score at his normal rate. And even when he does do that, he's still not got the passing/link up ability, or speed that Jesus has.
His goals have all come against poor teams and he's got 1 assist in 24 games - compared to Jesus' 2 in 2.
Perhaps this lack of goalscoring edge is because he's focusing so hard on the new stuff that he's not being natural in front of goal, who knows - but his effort isn't the problem, hasn't been for a while now.
You'd have noticed that back in August/September/October when Pep was actually unhappy with his workmate and what he was giving the rest of the team, he told us about it.
He would happily tell the press that Sergio needed to give us "more in the buildup", run more, work harder, but he's not saying that anymore, he knows Aguero is doing what he asks, he's just not doing the rest.