The striker departed Signal Iduna Park over the summer and scored the winner in his first game against his former employers on SaturdayJulian Schieber returned to haunt Jurgen Klopp as he scored the only goal in Hertha Berlin's 1-0 win over struggling Borussia Dortmund.
The striker, who spent two years at Signal Iduna Park before joining Hertha in the close-season, danced around the Dortmund defence before slotting a powerful finish high into the net in the first half.
Although an estimated 15,000 travelling supporters urged Jurgen Klopp's side on during the second period, they struggled in their attempts to salvage a point.
Dortmund dominated after the break but found Hertha goalkeeper Thomas Kraft in fine form, the former Bayern Munich man routinely repelling the hosts.
Ciro Immobile should have scored late on, but his gilt-edged opening slipped away to leave Dortmund back in the relegation play-off place as their woeful campaign domestically goes on.
Mitchell Langerak started a third consecutive game for Dortmund ahead of the out-of-sorts Roman Weidenfeller, but the Australian would have been helpless to prevent Hertha taking the lead had Schieber managed to connect with Anis Ben-Hatira's cross.
Luhukay's men almost had the lead in the 26th minute when Ronny was felled just outside the box, but his resulting free-kick drifted just over the crossbar.
Matters took a turn for the worse for Dortmund soon after when the lively Mkhitaryan limped off with an apparent hamstring injury, with Jakub Blaszczykowski making his first appearance of the season in the Armenian's place.
But the Polish winger was left red-faced when, five minutes before half-time, Per Ciljan Skjelbred robbed him of possession and slipped in Schieber, who kept his nerve to move the hosts ahead.
Dortmund went close through Pierre-Emerick Aubameyang as they pushed for an instant leveller, but Hertha could have easily extended their advantage had Langerak not saved well from Roy Beerens' drilled effort.
Klopp made a change at the break, introducing Hertha's top scorer of last season, Adrian Ramos, in place of the ineffectual Sebastian Kehl, though the Colombian forward was unable to settle as the home team continued to suppress the Dortmund attack.
And Hertha should have gone further ahead when Ben-Hatira broke into the box, only to fire into the side netting from close range.
Kraft routinely frustrated the visitors, saving impressively from Mats Hummels, Marcel Schmelzer and Gundogan as Hertha held on.
Immobile wasted Dortmund's best chance of all in the dying embers, heading wide when totally unchallenged from eight yards, condemning Klopp's side to their ninth league defeat of the campaign and dropping them back into relegation danger.