The Norway international was missing in action as the Bees secured an impressive 3-1 victory over Newcastle United at Gtech Community Stadium.
"Kris picked up a spasm in his back yesterday (Saturday) in training," Andrews told the club website.
"He felt something a bit awkward and it wasn’t great when he woke up today so he wasn’t in the squad."
The former Republic of Ireland international also talked about his team’s victory over the Magpies.
"It was thoroughly deserved; it was an outstanding performance,” he continued.
"We maybe edged the first half without creating too many clear-cut opportunities - I would say we looked the more likely.
"Half-time, going a goal down, I just wanted us to remain calm and focus on what we talked about before the game.
"There were a couple of tweaks on how we could improve things... I thought we were outstanding in the second half, and our performance levels went through the roof."