Armstrong is the latest young talent to emerge at Tannadice, with the likes of Ryan Gauld and Andrew Robertson securing moves away from United to Sporting Lisbon and Hull City respectively this summer.
The Scotland Under-21 captain, who recently earned a call-up to the full international squad, is regarded as one of the best midfielders north of the border.
The 22-year-old was named in the 2013–14 Premiership PFA Scotland Team of the Year and also nominated for the Young Player of the Year award.
Despite being only 22, Armstrong has made over 120 appearances in Scotland's top flight and he is tipped for a bright future in the game.
Armstrong's displays have not gone unnoticed south of the border with the likes of Southampton, Burnley, Wigan all keeping tabs on the goalscoring midfielder.
United are loathe to lose Armstrong this summer, but are bracing themselves for offers for the midfielder before the close of the transfer window.