Throwing on a center forward for a defender or midfielder while down a goal used to be a very commonly employed tactic by teams at all levels. It's been phased out a bit at the highest level in recent years, mostly because it doesn't work that well. Teams of all kinds are a lot better at playing keepaway against a team that's outnumbered in midfield than they used to be. It's not rare to see second division sides knocking the ball around like Barcelona against opponents that don't have a significant midfield presence.
Paris Saint-Germain are good enough to do this against just about anyone with a weak midfield, but they were awful at keeping the ball and winning it back from Chelsea all game on Tuesday, so Jose Mourinho decided to take a calculated risk. Searching for a goal, he threw on a forward for a midfielder. Then he did it again, essentially putting his team into a 4-1-5 formation. Incredibly, it worked.
While dominating the game, but in need of a finishing touch that hadn't yet come, Mourinho threw on Demba Ba for Frank Lampard, moving Oscar into the center of midfield. It was a sensible move — PSG didn't look like they were going to start winning the midfield battle, and Oscar is perfectly capable of playing in a two-man center. That didn't quite get the job done, so Mourinho made another similar move, putting on another forward.
Enter Fernando Torres, who hadn't scored in his last eight games. He's been poor recently, even by his significantly reduced standards. He entered for Oscar, leaving Chelsea with one central midfielder — a converted central defender. One midfielder, two very attack-minded wingers, three strikers. And, again, one of these strikers was Torres. It seemed crazy, but John Terry said after the game that Chelsea had a plan all along.
"We worked a lot all week on scenarios — 1-0, 2-0, 3-1, what would we do if Demba came on," said Terry. "We planned to hit the big man, and he has scored a great goal. For every scenario, we had a gameplan and once again we got it right."
Chelsea's tie-winner was exactly the type of goal one expects to see scored by a team that would employ Mourinho's tactics. Eto'o had a shot blocked, Cesar Azpilicueta had a follow-up effort deflected and Ba was in the perfect spot to clean up the trash in the box and send Chelsea through to the semifinals. It was probably the exact type of goal that Mourinho envisioned when he made his substitutions.
PSG aren't without criticism here, of course. They had three central midfielders on and their wide players were more attacking midfielders than wingers. Their center forward, Edinson Cavani, is pretty good at dropping deep to help his team keep the ball and start counters. Even when Lucas Moura came off for Marquinhos — a move made to counter Chelsea's three strikers — PSG should have been able to keep the ball pretty well.
Somehow, they couldn't, which is why the throw a bunch of strikers on trick usually doesn't work. It leads to the other team being able to keep the ball pretty well if their opponents take off midfielders, or create huge chances if they take off defenders. But PSG hadn't been able to create chances or keep the ball all game, so it was worth the risk for Mourinho.
It might be a long time before we see this trick work at a level this high again. A team with guys like Cavani, Thiago Silva, Yohan Cabaye, Blaise Matuidi and Thiago Motta on the pitch should be able to kill off a game where a team sacrifices midfield and/or defense for pure center forwards. But because they couldn't, Mourinho looks like a genius for using a seemingly outdated tactic.