I think it's the other way around. In a 4-man defense, he was a liability at FB, but in a 3-man defense, he was excellent at getting forward and intelligently picking out people or even getting to the back post (first goal in the FA Cup SF against Man City). Good thing is that Wenger sees the 3-man defense continuing, but Monreal and Kolasinac would be great rotation at LWB, which is a position which requires significant rotation anyway and hence, 2 players of similar calibre.