The Norwegian said: “We need to improve. They are a good team. I saw them play the first leg at home against the team from Montenegro and it is a big step up from what we have met in Iceland.
“In the second-half there was only one team on the pitch and we won quite easily.
“We didn’t want to concede an early goal, but it happens sometimes. We came back and 4-1 is a positive result and I am looking forward to the next round.”
Deila was critical of Stjarnan’s artificial pitch, which he claimed was not watered due to a broken pipe.
The Celtic boss said: “It is not a problem to play on an artificial surface, but when you don’t water the pitch it is totally different, everything is very sticky and everything goes much slower than usual.
“I always ask to water the pitch, but the water pipe was broken this time – maybe it is ready again now.
“But that is how it is. We have to handle every circumstance and surface you play on and we made it, we are through to the next round.”