The top of the Bundesliga table has been transformed since the final day of last season. Bayern Munich ended the campaign – and Jupp Heynckes ended his managerial career – with a surprising 4-1 home defeat to Stuttgart, yet they still finished a massive 21 points clear at the top as they won their sixth successive league title. Bayern are now down in fourth, a full nine points behind current leaders Borussia Dortmund.

Dortmund have come a long way since that day in May, when they nearly blew their chances of playing in the Champions League this season. When Pavel Kaderabek put Hoffenheim 3-1 up against Dortmund with just over 15 minutes remaining on the final day, Dortmund’s goal difference was just one better than that of Bayer Leverkusen – who were directly below them in the table, level on 55 points each, with a 3-0 lead over Hannover at the time. Had Leverkusen found another goal they could have leapt over Dortmund and into the final Champions League spot, but two late goals from Hannover – in the 94th and the 96th minute – eased Dortmund into fourth.

Dortmund needed a new approach this summer and started by ushering manager Peter Stöger out the door. In his place came Lucien Favre, who returned to the country where he had worked wonders with both Hertha Berlin and Borussia Mönchengladbach. The Swiss coach took Hertha Berlin from 10th to fourth in his two years in the capital before saving Borussia Mönchengladbach from relegation in 2011 and securing another fourth-place finish the very next season.

In Favre’s final full campaign with Gladbach, in 2014-15, the club finished third in the Bundesliga and qualified for the Champions League. His time did come to an end after five straight defeats at the start of the following season, but Favre left Germany with a very strong reputation. Dortmund had high hopes when they asked him to rebuild their team this summer, but no one could have anticipated this sort of start.

Paco Alcácer scores another goal for Borussia Dortmund. Photograph: Friedemann Vogel/EPA

Bayern Munich have been unusually frail this season under their new coach, Niko Kovac, and Dortmund have exploited that weakness fully, storming into a seven-point lead at the top of the table, with a nine-point advantage over Bayern. Dortmund’s goal difference of +24 after 13 matches is far by the best in the division, which suggests they are blowing teams away, but their success is actually based on their ruthless efficiency in front of goal.

Their strength is not in creating a lot of chances, but in taking them. Two big wins – their 7-0 thrashing of lowly Nurnberg and their 4-0 victory over Stuttgart – account for almost half of their goal difference. In general, they have not been overly dominant in matches. In fact, they have taken 56 fewer shots than Bayern and just one more than last-place Fortuna Düsseldorf. Dortmund are top of the league because of their tremendous finishing.

Dortmund have scored from 22% of their shots this season, the best chance-conversion rate in the division and almost double Bayern Munich’s (11%). That accuracy in front of goal is largely down to one man: Paco Alcácer. The Spaniard has been a revelation since joining the club on loan from Barcelona in August – a deal that has since been made permanent. He is the Bundesliga’s joint top scorer with 10 goals even though he has started just two league matches.

He was not in the starting line-up for Dortmund on Saturday in their 2-0 victory over Freiburg but, yet again, he came off the bench and scored. That was his ninth goal as a substitute so far this season – enough to break the Bundesliga record in a single season, and all by the first day of December. On top of that, not a single goalkeeper has saved one of his shots this season. If he has hit it on target, it has gone in.

Paco Alcácer has scored league 10 goals from two starts this season. Photograph: Friedemann Vogel/EPA

So why is Alcácer not starting? After all, Dortmund don’t have another natural No 9 to keep the Spaniard out of the side. Maximilian Philipp has led the line on six occasions this season but he played predominantly from the left flank last season. Even Mario Götze has been picked up front more often than Alcácer in the league this season.

Favre’s reluctance to pick his in-form goalscorer is partly due to concerns over his fitness in the past, having spent two years warming the bench at Barcelona. Perhaps the coach is also of the opinion that if it ain’t broke, don’t fix it. His team are still unbeaten after 13 games – 10 of which they have won – and, when playing as a substitute, Alcácer is scoring a goal every 21 minutes when including injury time – within which he has now scored three.

More than half of Alcácer’s league goals (six of his 10) have come in the 80th minute or later, while his tally has directly earned Dortmund nine points – fittingly, the same number as their lead over Bayern. The summer signing has made the difference for Favre, but he may have to continue to make do with a place on the bench.

The striker has the rare problem of being too effective as a substitute to start games. As matches open up, Dortmund’s bright attacking talents are given more space to exploit and Alcácer can come on and be the perfect finisher of both goals and games. He fits the system perfectly at present and, while his statistics will undoubtedly prove unsustainable, the title chasers have no reason to rush their in-form frontman into the starting line-up.

• Follow WhoScored on Twitter and Facebook

• Follow Martin Laurence on Twitter