Stark, a former Celtic midfielder himself, worked with Armstrong at the Scotland Under-21 national team.
The former Kilmarnock and Hamilton Academical midfielder has revealed the advice he gave to the 24-year-old ex-Dundee United man.
“I used to tell him a midfielder with the qualities he had must be scoring double figures every season,” Stark told The Scottish Sun.
“He accepted that because he strikes a nice clean ball. He can arrive late in the box and get one-touch finishes.”
As reported by The Scottish Sun, Armstrong has scored eight goals for Celtic so far this season.
As stated on Celtic’s official website, the former Scotland Under-21 international has scored 14 goals in 85 appearances in all competition for the Bhoys since joining them from Dundee United in February 2015.
Brendan Rodgers’s side are at the top of the Scottish Premiership table at the moment with 58 points from 20 matches.
The Hoops are 19 points clear of second-placed and Old Firm rivals Rangers, who have played a game more.
Celtic will return to action on January 22 when they take on Albion Rovers away from home at the Exsel Group Stadium in the fourth round of the Scottish Cup.