The Spanish international agreed to join the Gunners on Thursday after first-choice left back Kieran Gibbs was ruled out for up to six weeks with a thigh injury.
Santos received a mixed reception from the Emirates Stadium crowd when he replaced Gibbs during the midweek draw against Liverpool but Wenger defended the Brazilian ahead of the visit of Stoke City.
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"Maybe people speak that he had a difficult game [against Liverpool] but he lacks competition," said the Arsenal manager.
"He is a style of player who needs competition because he is not a nervous type of player, he is a stamina type of player and get sharper through competition.
"Yes [the criticism of Santos is unfair] because if you watch the game again and you analyse very well his performance, you will be less critical.
"The first critics after the game are always a bit emotional because you have one or two big mistakes in your mind and we are all the same on that front. But when you watch the games again, you will see that he has done some good things."
Wenger moved quickly to sign Monreal after hearing that Gibbs' spell on the sidelines would stretch beyond a month. But the Frenchman will give Santos his chance - with Bayern Munich a likely opportunity.
"Honestly, I believe that Kieran Gibbs will be out for the Champions League because we are only three weeks away from Bayern now, not even three weeks and I don’t think he will be available.
"I will not play Monreal [because he is cup-tied] and certainly not Gibbs, but if you want me to tell you who will play, today I can't tell you. Vermaelen can play there as well and Santos could have a key role as well, yes.
"I believe we need two left backs," added Wenger. "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."Copyright 2013 The Arsenal Football Club plc. Permission to use quotations from this article is granted subject to appropriate credit being given to www.arsenal.com as the source 1 Feb 2013