The Tannadice side are 13 points adrift of second-bottom Kilmarnock and in turmoil following the 3-0 home defeat to Motherwell on Tuesday night.
United chairman Stephen Thompson subsequently apologised for their form and told the players that they have "13 matches to preserve the club's Premiership status and redeem their own professional reputations"
Armstrong who, along, with Nadir Ciftci and Gary Mackay-Steven, was sold to the Parkhead club from United in 2015, was not aware of Thompson's scathing statement but knows the predicament the Taysiders are in at the moment.
The 23-year-old Celtic midfielder said: "It's very sad to see
I was hoping they would get a result on Tuesday night but it wasn't to be and their future isn't looking good in this league
"But you have to remain optimistic and hope they can finish the league strongly and then, if they do come down, they come back up as soon as possible."
Source : PA