Armstrong signed a new two-year contract with Celtic earlier this month which will see him remain at the Glasgow giants until the summer of 2019.
The 25-year-old midfielder has been on the books of the Celtic Park outfit since February 2015 when he joined from Scottish Premiership rivals Dundee United.
The Scotland international played well last season, and helped Brendan Rodgers’s side win the Scottish Premiership, the Scottish League Cup and the Scottish Cup.
Before he signed the new contract, Armstrong was linked with moves away from the Glasgow giants in the summer transfer window, with West Bromwich Albion, Swansea City and Southampton reported to be interested in him by The Daily Mail.
The 25-year-old Scotsman has revealed that negotiations over a new deal did not affect his game, and has also claimed that he always knew that he was going to remain at Celtic Park.
“In my head it was clear that I was staying so there was no issue with being unsettled,” Armstrong told The Telegraph. “It was obviously just everything surrounding it – I just wanted to focus on playing football.
“It was always my decision to get that contract signed. It just took a bit of time but now that is all sorted and everything has settled down.
“Everyone is fighting for a spot to play for Celtic and that’s the way it should be. It has been that way since the day I joined the club and it is no different now.”
"The only top-class defender they've got is Tierney,” Stubbs told BBC Sport. "Domestically, Sviatchenko, Boyata, [Jozo] Simunovic are competent, but the Champions League is a whole new ball game.”