Looking back now it seems crazy, but when I first trained with Sergio Aguero after he joined Manchester City in 2011, I did not instantly think 'superstar'.

Sometimes when a new player arrives at your club, you see him in action with your own eyes and just think 'wow'.

That was happening a lot at City at the time, because of the kind of big signings we were making that were transforming the club, but it was not the case with Sergio. When I first saw him, I didn't actually think he was that good.

We had spent a lot of money on him - around £35m - and he had arrived with a big reputation, but in those first few sessions he did not look particularly special.

It is not as if I was expecting him to be dribbling past four or five players or anything, but I was marking him a lot of the time and he did not really do anything at all.

Pep Guardiola congratulates Aguero on 'incredible achievement'

He looked lethargic and was actually quite lazy if I am being totally honest. I was wondering what all the hype was about.

I did not realise just how good he really was until he played his first game for us. He came alive in matches, and everything about him was different.

I actually set up his first goal for City, on his debut against Swansea, with a cross that he turned in at the far post - and he has obviously scored quite a few more like that since.

When I look at his record now, as the highest scoring overseas player in Premier League history, I wonder why I ever disrespected him.

His achievements are phenomenal, and they just keep on coming.

Aguero surpassed Thierry Henry as the highest-scoring overseas player with his hat-trick against Aston Villa on Sunday

Movement just part of what makes Sergio so special

What is so special about Sergio? Where do you start? Everything about him is world class.

His movement when City are attacking is just incredible. His game is not just about running off defenders, because he can come deeper to get the ball too. Either way, he manages to find space when there should not be any there.

Then there is his finishing. If the ball drops to him in front of goal, then I would back him to score every single time.

That is partly down to him being so calm when he gets a chance, but also because of the way he strikes the ball so cleanly - put all that together, and you have a deadly finisher, and it all comes so naturally to him.

Some strikers see their goals dry up as they get older, but not Sergio. He is 31 now but he has sustained his minutes-per-goal ratio for several seasons.

Aguero has scored a Premier League goal every 106 minutes since joining City in 2011. This season he has the highest shot-conversion rate (25.49%) and best minutes-per-goal ratio (a goal every 77 minutes) of any campaign during his time at the club.

Yes, it helps that he is at City and playing for a team who create a lot of chances, but he has kept on working hard to ensure his levels have not dropped, even with the amount of injuries he has suffered.

I would put that down to Aguero's character, which is another big reason he has been successful.

When Pep Guardiola took charge at City in 2016, Aguero was not always playing but he reacted in exactly the right way.

While he might have shown his frustration on the pitch when he was being substituted, Aguero has never complained publicly about anything off it.

I think that is just because he has always backed his own ability. His approach seemed to be along the lines of 'well, you can play whoever you want in front of me, but when I get to play again, I am going to score'.

He is willing to listen too, which is how he has been able to adapt his game under Guardiola.

You saw another example of that in City's win over Aston Villa on Sunday. Aguero got another hat-trick, of course, but you also saw him retrieve the ball to help set up his side's second goal.

When I played with him, he was just more of a finisher. The work ethic you see from him now is what Pep has instilled in him and that is why he has taken his game to the next level because now he is doing all the stuff he did when I played with him, and more.

The medal that would put him up there with football's greats

Aguero has another match-ball to add to his collection after his hat-trick in City's 6-1 win over Aston Villa on Sunday, his 12th in the Premier League - another record

Replacing Aguero, when that moment comes, is going to be near impossible for City.

His goal-scoring record is just incredible, and so is his attitude. For someone who is a genuine superstar, he is so humble - and he is the nicest man you will ever meet.

A lot of superstars might be great talents on the pitch but can also be a bit difficult off it. With Aguero, both parts are spot on.

Yes he shows emotion, as he did with Riyad Mahrez at half-time at Villa Park, asking him why he didn't pass to him when he was clean through, but it is only ever football-related.

When you talk to him, he just wants to play, and he just wants to score goals. That's what he is all about, and that is what I love about him.

Aguero played in City's first Champions League game, against Napoli in 2011. This is his ninth campaign with the club in that competition - so far, the furthest they have got is the semi-finals in 2016.

With his talent and attitude, it was inevitable he would have so much success in the Premier League, and of course he has his moment in history here already because his goal that won the title in 2012 will never be forgotten.

But when he does leave the club, I just hope he has got a Champions League winners medal too, because that is the only thing he is missing at club level.

Whether he wins it or not is not going to make or break his legacy as City's greatest ever striker, or his reputation as a player, because he has already achieved so much. But it would put him right up there with the greats, which is where he deserves to be.

Can City triumph in Europe this year? Well, they are certainly good enough going forward to win it, and Aguero is just one of the reasons for that.

My concern is that teams like Real Madrid and Paris St-Germain - to name but two - have got world-class forward lines too.

Sometimes, when I see City play, because they are so expansive and play with such freedom, they cannot help but leave gaps.

They cannot afford to do that now they are in the knockout stages, where one bad half can be enough to knock you out, and defensively they need to be more solid if they are going to go all the way.

Micah Richards was speaking to BBC Sport's Chris Bevan.