Armstrong is one of the best midfielders in the Scottish Premiership and was in brilliant form for Celtic last season.
The 25-year-old Scotland international played a very important role for the Bhoys during the 2016-17 campaign, as Brendan Rodgers’s side won the Scottish Premiership, the Scottish League Cup and the Scottish Cup.
Former Celtic striker Sutton has suggested that he is worried about the form of Armstrong, and hopes that it improved soon.
“All I’m saying is there are a couple of worrying signs that I’d like to see addressed before they get there,” Sutton wrote in The Daily Record.
“One being the form of Stuart Armstrong. Although it’s early days, he’s looked a bit off it and this situation with his contract needs sorting out one way or the other because it appears to be having an effect on him.
“He wasn’t anywhere near at the level against Rosenborg and he needs to focus on his performances on the pitch. He owes his team-mates and Brendan Rodgers that.”
Armstrong has been linked with moves away from Celtic in the summer transfer window, with clubs in the Premier League claimed to be interested.
According to The Scottish Sun, Southampton and Brighton and Hove Albion want the 25-year-old midfielder, while The Birmingham Mail has credited West Bromwich Albion with interest in the former Dundee United star.
Meanwhile, James Forrest has revealed what manager Brendan Rodgers told the players after the win against Rosenborg in the Champions League qualifying third round tie last week.
“The gaffer said after the game that he had watched Celtic away from home in Europe and we had a tendency to sit back,” Forrest told The Scottish Sun.
“So he was delighted that we all wanted the ball and we looked the stronger team and took the game to them, away from home.”