I finally have the book handy and I re-read some parts. Watanabe calls Midori from "the place that is no place" at the end of the novel. I think it is clearly meant to contrast with the scene at the beginning of the novel where Naoko and Watanabe are walking through a sunny field by themselves. Watanabe explicitly states that he remembers the scene quite well except for the people: himself and Naoko. So here he senses all "background" (grass, sky), but no "foreground" (himself, Naoko). But in the phone call scene he senses foreground (himself, Midori) but no background (we don't know where he is).
Murakami just gives the contrast without explaining it. If he did explain, well, then that would be its meaning. But since he didn't, I guess the contrast could mean different things. For me, I guess that it means that he did fall in love with Midori, and he fell out of love with Naoko. Or maybe more accurately, he suppressed his memories of Naoko in order to survive until the natural human process of forgetting took over.