The 23-year-old central midfielder started the Scottish Premiership clash at the Firhill Stadium in Glasgow, but he was taken off after just 21 minutes.
Deila has explained that the Scot was substituted due to an injury and has admitted that he should not have featured in the game.
“Stuart Armstrong was a stiffness in the groin, so we weren't taking any chances,” the Norwegian told BBC Radio Scotland.
"We wanted to start with him, but that was not a wise decision - but no harm was done."
Two wins out of two
Celtic made it two wins out of two in the Scottish Premiership with a 2-0 victory over Thistle away from home.
Tom Rogic put the Bhoys in the lead at the Firhill Stadium just before the half-hour mark, and Kris Commons added a second in the 63rd minute.
Over the course of the 90 minutes, Celtic had 59% of the possession, took 15 shots of which six were on target, and earned five corners.
Meanwhile, hosts Thistle had 41% of the possession, took five shots of which just one was on target, and earned one corner.