The German winger struck with 20 minutes remaining of a keenly-fought encounter to give the Foals a winning start to the new year.
Gladbach on top
Following an impeccably observed minute's silence in memory of late VfL Wolfsburg midfielder Junior Malanda, the contest got off to a furious start. Gladbach defender Martin Stranzl headed over as Sven Ulreich flapped at a Max Kruse corner, although the Stuttgart goalkeeper redeemed himself moments later with a convincing save to keep out a fierce drive from the Austrian centre-back.
It was all Gladbach in the opening exchanges, save a rare Stuttgart foray involving Martin Harnik. Branimir Hrgota should have done better with a pin-point Oscar Wendt cross, before Stuttgart allowed US international Fabian Johnson to run inside unchallenged and fire narrowly wide of Ulreich's left-hand post from a good 20 yards.
After a slow start, Stuttgart began to find joy out on the flanks, with captain Christian Gentner's miscued cross grazing the top of the bar and Yann Sommer at last called into action to foil Timo Werner. Gladbach's Hrgota spurned the chance of the half, though, heading over from Kruse's delivery when it looked easier to score.
Worryingly for the Foals, ten of the 16 league goals they'd conceded prior to Saturday's clash, as well as 15 of Stuttgart's 20 Hinrunde strikes, had come in the second half. The hosts really ought to have added to that tally in the 51st minute, but Georg Niedermeier somehow contrived to lift Timo Baumgartl's flicked header over the crossbar from two yards.
Herrmann springs to life
There was the distinct feeling one goal would be enough to win it and, after Baumgartl rode to die Schwaben's rescue to deny Hrgota, Gladbach finally got their noses in front. Hrgota's square ball released Herrmann, who took the bouncing ball in his stride and sent a venomous effort into the roof of the net for his third league goal of the campaign.
Borussia head coach Lucien Favre cut a relieved figure as the full-time whistle blew, having just witnessed the luckless Niedermeier strike the crossbar deep into added time. The result represents the Foals' first away league win since 18 October and plunges Stuttgart back into the bottom two.
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