Arsene Wenger felt his side became complacent before Burnley’s equaliser made their FA Cup clash a “different game” at the Emirates.
Calum Chambers gave the Gunners a 19th-minute early lead in their fourth-round tie but Sam Vokes brought the Championship side level on the half-hour mark.
The holders secured a place in the last 16 when Alexis Sanchez notched the winner on his first start since returning from injury.
Wenger admitted the victory was not as comfortable as the hosts thought he might have been during the first half.
He said: “We needed to work. At the start it looked like it would be easy because we had a lot of the ball and good opportunities around the box.
“When it came back to 1-1, it became a different game. I think we were a bit shocked and they gained belief.
“I saw them against Derby and I could see that this is a very efficient, strong team with no weakness. Everything they do is based on efficiency and they’ve shown that today.
“In the second half, I don’t think they created a lot. They had one or two opportunities on the counter-attack but we managed to win the game and we looked quite in control at the end. We needed to keep our focus, our urgency until the last minute.”
Wenger admitted he had some doubts about handed Sanchez a start against the Clarets.
“When I saw Derby against Burnley, I had a little hesitation to pick the three [Sanchez, Francis Coquelin, Mohamed Elneny]. But the fact that we played at home, I thought we could manage to get away with it.
“Alexis I was not too much worried about on the fitness front because I kept him out two weeks ago and he was already there physically. We had some worries about his muscle but today he came through well and you could see that physically, he is ready.”
New signing Elneny was also given a start and Wenger felt the midfielder settled in nicely.
The boss said: “I felt he started a bit cautious, played a bit secure. He became more adventurous, he had some good shots on goal. His mobility, his work rate was exceptional and for the rest, he needs to adjust to the power of the game in the Premier League.
“This was a good welcome today and certainly he has learnt a lot today. It will take him some time to adjust to the power side of our game here, but the intelligence, the mobility and the technical level are good.”