The Celtic midfielder, 23, hopes the Tangerines, who are 13 points adrift at the foot of the Premiership, can avoid relegation.
United chairman Stephen Thompson has challenged his squad to preserve their top-flight status and "redeem" their professional reputations.
"It's very sad to see," Armstrong said. "Their future is not looking good in this league."
Armstrong was one of three players United sold in to Celtic in 2015. Since his departure - along with Nadir Ciftci and Gary Mackay-Steven - United's form has slumped.
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Mixu Paatelainen's side lost 3-0 to Motherwell at Tannadice on Tuesday and Thompson then apologised to the club's supporters for their "abysmal" display and position in the table.
"I was hoping they would get a result on Tuesday night," said Armstrong. "It was not to be.
"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."
Meanwhile, St Johnstone boss Tommy Wright believes no-one should be writing off the Tangerines until it is arithmetically impossible for them to stay up.
"I would never say anything is done until it is done." Wright told BBC Scotland.
"They are not in a great position, that is an obvious thing for me to say.
"What they have got is a difficult position to get themselves out of. It shows the change in Scottish football."
Scotland's top flight has lost Rangers, Hearts and Hibernian in recent years, though Hearts have since returned to the Premiership and Rangers and Hibs are trying to get promoted to the top tier for next season.
Wright admits it would be "disappointing" for Saints to lose one of their Tayside rivals out of the league.
"We always enjoy the derby games with them," added Wright.
"Rangers or Hibs would probably be an automatic replacement but everybody wants to see the top clubs in the league on merit."
Dundee striker Greg Stewart would also miss local derby matches with United but concedes his own club's supporters may not feel the same way.
"Personally I enjoy playing in the derbies but I suppose Dundee fans won't agree with me," said the 25-year-old. "I'm sure they are wanting them down just to get the bragging rights in the city.
"The atmosphere in the games is brilliant for players and it makes you more determined and you want to win them.
"We have one more at least this season and hopefully we can make the most of it and get the win."
Stuart Armstrong is saddened by the plight of former club Dundee United.