However, Armstrong's early performances this season suggest that United might have made a huge mistake in selling the forward.
The top-flight side signed Salomon Rondon on loan as their new number nine this term, but Armstrong has been exceptional up to now with his new side and Rafa Benitez's team could have used his goals had they kept hold of him.
Armstrong netted two fantastic goals in Rovers' 5-1 first-round EFL Cup win against Carlisle United on Tuesday evening and has looked a threat in the league also when he has came off the bench for Tony Mowbray's side.
The former Toon striker has scored 36 goals for the four different clubs he has been on loan at since his time on Tyneside, showing he certainly possesses ability in front of goal. However, Armstrong has never scored in the top flight, despite being prolific in the lower leagues.
58-year-old Benitez will have to hope that new signings Rondon and Yoshinori Muto can score the goals the Magpies need, although neither managed to find the net on their debuts off the bench.
And the black and white faithful will be hoping that the cheap sale of Armstrong to Championship Blackburn won't come back to haunt them in the future.