For the first leg of the historic Copa Libertadores final between Boca Juniors and River Plate, one Argentine radio station hit upon an interesting idea for its transmission: The commentators maintained a calm tone of voice, while soothing music played in the background and there were frequent spots informing listeners how to get in touch with a cardiologist.

More of the same will be needed for Saturday's return game, with the difference being that this time there are no second chances. At the end of 90 minutes -- perhaps 120, maybe even penalties -- one of these Buenos Aires teams will have won the biggest battle in the history of a desperately intense footballing rivalry; the other will withdraw to lick its wounds and prepare itself for years of taunting.

The tie could not be closer after the first leg finished as a 2-2 draw, especially because the away goals rule is not in operation for the final and so neither team has an advantage to defend. The game, then, will be a test of incentives: Do you go out and seek to win or play tight and try not to lose, perhaps sneaking a goal on the break?

More of the same quality of the first game, on Nov. 10, would be welcome. It was an open encounter, full of ebb and flow, and has since been recreated using Lego. The level of tension and drama in the return match means it might have to be recreated with a mixture of UFC and opera!

Much of the credit for the flowing nature of the opening leg, which took place at Boca's Bombonera, must go to River coach Marcelo Gallardo, who surprised both with his tactical formation of three centre-backs and usual left winger Gonzalo Martinez in a central role, as well as his bold approach.

The return from injury of veteran holding midfielder Leo Ponzio for Saturday is a boost and could inspire Gallardo to attack more. However, his quickest striker, Rafael Santos Borre, is suspended, and a natural substitute, Nacho Scocco, is a major injury doubt. With a potential lack of attacking mobility, one solution might be to push Martinez further forward.

In more than four years in charge of River, Gallardo has developed the capacity to use different formations, and he has been looking at several in training this week. One thing is certain: He will keep his opponents guessing as to his true intentions until kickoff. Not that he will be alone in that regard.

Boca manager Guillermo Barros Schelotto also has options and will either line up in a 4-3-3, the way his side started the first leg, or in the 4-4-2 to which they successfully switched after Cristian Pavon limped off. The winger is struggling to be fit, and his absence at the Estadio Monumental would be a setback.

The visitors do have Sebastian Villa, whose pace and power out wide have been especially eye-catching in away games, plus plenty of other attacking options. There is Colombian playmaker Edwin Cardona, the strong Ramon Abila and in-from Dario Benedetto at centre-forward, as well as the choice of Mauro Zarate and Carlos Tevez to operate just behind.

On the evidence of the first leg, Schelotto has performed a fine piece of man-management with Tevez. The former Premier League and Juventus star has not been a success on his return to Boca, which was disappointing during the group phase in no small part because he was poor.

The team was too dependent on a veteran who seemed to lack the pace and sharpness to play a prominent role. Schelotto lost patience and dropped Tevez, who thereafter would not even get into games off the bench.

The 34-year-old could have been a disruptive influence but instead responded to the clear incentive of his boss: Keep working, get fitter and you will play a part in the final. Tevez played the last 20 minutes of the first leg and duly came up with his best display of the year to almost inspire victory.

A big-occasion player, he will surely have some role to play on Saturday, and if events follow his script, the evening will end with him and his teammates performing a lap of honour in front of a crowd filled entirely with River Plate fans.

For Boca, no triumph could be bigger, whereas for River, meanwhile, losing the South American title at home to its fiercest rivals would be the ultimate humiliation. On the other hand, the alternate outcome would start the biggest party in club history.

Everything is to play for, and nothing is certain except that when game time arrives Saturday, it is one or the other; no middle ground is possible.