Wenger has offered hope to Brazilian left back Andre Santos who will now be third in line behind Monreal and Kieran Gibbs when the England defender returns from injury.
Wenger added: "I believe we need two left backs. You cannot play every single game with the same player. We had two before, we have two again now.
"If Gibbs is out for four to six weeks, from six it can easily go to eight [before he is competitive again] and that means two months in a very decisive period. If’s vital to have another left back."