A deflected strike from Alex Mighten was the difference as Rovers suffered defeat against Nottingham Forest at The City Ground.
Rovers did have their chances after the match-winning moment with Armstrong crashing a shot against the post and then seeing a second-half penalty beaten away by Brice Samba.
Mighten's attempt, which deflected off the boot of Lewis Travis, deceived Thomas Kaminski and flew into the back of the net midway through the first 45 minutes.
Rovers improved after the break and worked some opportunities but couldn't find a way past Samba and earn what would have been a deserved point on the road.
The boss made five changes to his starting XI with recalls handed to Ryan Nyambe, Harvey Elliott, Amari'i Bell, Jacob Davenport and ex-Forest man Ben Brereton in the East Midlands.
A lightning start saw the first chance of the game fall the way of the returning Brereton, but his header was tame and comfortable for Forest's Congolese goalkeeper to claim.
The same player tried his luck with a curling effort soon after, but Samba simply shifted to his left to grab the goal-bound effort which was hit from outside the area.
Forest eventually made a move forward and Rovers were grateful for Nyambe, who covered brilliantly to head Anthony Knockaert's curling effort off the line after the French flanker left both Davenport and Jarrad Branthwaite on the floor.
However, just as it looked like Rovers were looking fine, the hosts scored a fortuitous opener midway through the first half.
A high ball into the box was headed away, but well kindly for Mighten, who took a touch and unleashed a strike that took a wicked deflection off Travis before finding the back of the net.
It was cruel on Rovers, but things could have got worse soon after when Murray worked a yard but lashed just wide of the target from a tough angle.
Rovers went in search of a response but Travis' effort was straight down the throat of Samba after fine work down the left involving Bell and Elliott.
There was just a minute of the half remaining when Tobias Figueiredo's glancing header cracked the upright before Rovers worked their best opportunity of the game at the other end through Armstrong.
The skipper for the day turned Figueiredo inside-out before unleashing a thumping strike that looked destined for the top corner. The attempt beat Samba but it thundered against the post and bounced away to leave Rovers cursing their luck as they went into the break behind.
A half-time alteration saw Dack replace Sam Gallagher as Rovers readjusted things and went with a 4-2-3-1 formation with the talisman just behind Armstrong.
Dack's influence was growing into the half and he was involved just after the hour, which ended with Armstrong touching, swiveling and then hitting wide of Samba's post.
Forest hit the post with 20 minutes remaining as Cyrus Christie ventured forward and saw his low shot hit the base of Kaminski's right-hand post and substitute Lyle Taylor then hit wide.
Rovers had a great chance to level just a moment later when Samba brought down Nyambe in the box, but the goalkeeper redeemed himself by guessing right and saving Armstrong's well struck kick from the spot.
Rovers had plenty of possession and continued to probe for an equaliser, but Mowbray's men were left dejected and headed home with nothing to show for their efforts.
Nottingham Forest: Samba, Christie, Worrall (c), Figueiredo, Ribeiro, Garner, Cafu (Yates, 69), Knockaert (Blackett, 85), Krovinovic, Mighten (Freeman, 69), Murray (Taylor, 59).
Unused substitutes: Smith, Mbe Soh, Colback, Jenkinson, Lolley.
Goal: Mighten, 25.
Booked: Murray, 42, Garner, 68.
Rovers: Kaminski, Nyambe (Bennett, 90), Harwood-Bellis, Branthwaite, Bell, Travis (Rothwell, 85), Davenport (Buckley, 85), Elliott, Gallagher (Dack, 46), Brereton (Dolan, 64), Armstrong (c).
Unused substitutes: Pears, Downing, Douglas, Evans.
Booked: Harwood-Bellis, 2.
Referee: Keith Stroud.