When the draw was made in December, the Gunners were made overwhelming favourites to win and qualify for the quarter finals for the first time in five years.
But their chronic defensive frailties were exposed in the first leg as they lost 3-1 at the Emirates, conceding a goal in the last minute.
A brave performance in the second leg saw Arsene Wenger's men beat the French club 2-0 on the night but they exited the tournament on away goals after drawing 3-3 on aggregate.
And the young right-back has admission of complacency from the team could concern Wenger.
He told Marca: "In the Champions League maybe we should have done things better.
"Perhaps we underestimated Monaco."