"Don't make a mistake."

I was working at Leicester's crucial game against Everton last week and the match analysts for both clubs were sitting next to me. I said "look lads, put your monitors down - it's not about tactics tonight, it is just about avoiding any errors".

That was a Premier League relegation battle while this is a Champions League semi-final, but City versus Real is exactly the same scenario, just at a much higher level.

You can give players all the data you've got, and detailed instructions for how to play a particular role or deal with a certain opponent. All of that is important, of course, but they also know that if they suffer one lapse in concentration then the game has changed, and maybe even been lost.

That's what happened to City when they played Real in last season's semi-final. They were in full control of the tie heading into the 90th minute of the second leg… and they still didn't go through.

How do City keep Vinicius Jr quiet?

We know this Real team can score a goal out of nothing and, especially in the first leg at the Bernabeu, City have to be extremely careful.

The first game was very open when they met last year but, this time, we might see City start out by trying to be tighter defensively.

That means keeping Vinicius Jr quiet and I think the two big decisions Pep Guardiola has to make are who will go up against the Brazil winger on the right for City, and who will play further forward on the same flank.

The choice in defence is really between Kyle Walker and Manuel Akanji, but if Nathan Ake does not recover from the injury he sustained against Leeds on Saturday then it is possible Pep's mind will be made up for him.

He could go with the experience of Aymeric Laporte at left-back but I have a feeling he will pick Akanji because of how well he did against Bukayo Saka when he played there in last month's 4-1 win over Arsenal.

Ake did not train with the rest of the City team on Monday but, if he is fit, Guardiola has more options. He could use John Stones in his inverted full-back role, where he comes inside from right-back to become an extra man in central midfield, leaving Akanji to move out wide to the right to deal with Vinicius.

I think Guardiola would love to use that system at some point against Real, but he will be worried about their threat on the counter through Vinicius.

Walker's speed is the obvious way to deal with that and, as a natural full-back, I feel he has to be the answer there, whether Akanji is needed to cover for Ake or not.

Manuel Akanji has played 42 games for City this season but his only appearance at left-back was against Arsenal on 26 April, with Kyle Walker at right-back and John Stones and Ruben Dias in central defence

Akanji has been incredible since joining from Borussia Dortmund last summer and, at just £15m, has to go down as the bargain signing of this season.

He deserves to play somewhere in City's backline against Real but that is irrelevant because all Pep will consider is who is best suited for this specific game.

And when you are facing one of the most dangerous wide players in the world, why would you put a centre-back in that position when you have Walker, who is made for the job?

It's not just Walker's pace that makes him perfect, it's his power and his understanding of how to deal with the problems Vinicius poses, which he has picked up by playing in so many huge games in the past.

Walker's presence would be important psychologically too. From Vinicius's point of view, who would he prefer to face - a centre-back coming out wide, who is not as quick as him... or someone who he knows he cannot outrun?

Benzema's clever movement poses a threat

City's starting XI versus Arsenal in the 4-1 win on 26 April could provide some clues to how they will line up against Real at the Bernabeu on Tuesday

Stopping Real is not just down to the defenders, though. It starts from the front. If City pick Erling Haaland to lead the line, and it is pretty obvious they will, then he is not the best at pressing - they will need other players who provide that.

That makes me think Guardiola will go with Bernardo Silva over Riyad Mahrez on the right because, although Pep likes to have one quicker attacker in European away games, Bernardo will give the team more legs with his work-rate to press when they are out of possession.

It is that flank where Real are most dangerous, through Vinicius, so if you put Bernardo in front, then he can double up with whoever his right-back is.

Bernardo will play somewhere, I am sure. City will need his work-rate whether it is out on the right or in the centre of midfield alongside Rodri and Kevin de Bruyne.

I am expecting City to dominate possession but they will have to press Real in the middle of the pitch, where they might be outnumbered, because that is another area where the holders can do some serious damage.

City have to try to screen Karim Benzema and stop passes being played into him, but that is made more difficult by the fact he is constantly on the move and happy to get on the ball anywhere.

Karim Benzema found space to provide the assist for Rodrygo's second goal in Real's remarkable comeback against City last season

When Rodri was asked about Benzema last year he talked about how the Frenchman does not just score goals, he also makes Real feel more comfortable in possession. His movement is brilliant so they know they can always find him, and usually in space.

One of the City centre-halves will have to stay tight on him when he comes deeper to collect those passes, and looks to flick balls over the press and find runners like Vinicius.

That will be another constant danger in Spain and also in the second leg at Etihad Stadium next week.

City could win out there and still find it tough to finish the job and, to get through to the final, it feels like they will need to have everything going for them. I don't think anyone will be saying the tie is over, whatever the score is after the first leg.

Michael Brown was speaking to BBC Sport's Chris Bevan.