When trying to make a realistic style manga, there are a lot of ways to go about it. You could focus on some really heavy issues or you can start focusing on the cute parts more than the serious ones and follow the usual slice of life format. Ahiru no Sora is a mix of these 2 and sadly, it isn't a very good one.

Ahiru no sora has a lot of things it does badly than the other way around. When the best thing about the manga is the art then there is a very big issue here and I'll show you most of the things it did wrong.

First, the pacing. The manga has horrible pacing, a match takes way too long to finish that you could spend 40+ chapters in one match, it takes a lot of time between each match and it doesn't even use that time well. It mainly is about one of the characters getting character development out of nowhere in the middle of the match every single time.

Second, the realistic approach. This manga is supposed to be realstic but it just isn't done right. The idea of a team that was just made after years of no training and not playing together yet they all have that wonderful natural talent is just way too convenient. I liked that the mangaka makes it apparent that they should lose but playing every single game and losing right at the end everytime is just random. There is no severe beating and all games are close which is not realistic at all.

Third, the characters. Out of the top of my head, I only like 3 characters of the anime, Sora's mother, Sora and Nao. The rest are just fine or downright horrible.
For the good characters:
I really liked Sora's mother because of how well written she was and I also liked Nao and Sora because they were represented very well as hard working characters that actually display emotion and try to achieve something.

For the bad ones:
Chiaki is the genius guy who can do anything because of his talent and the same thing with Kenji. That was their whole personality and they get very little character progression.
Madoka is the type of girl that goes to your house, sucks you off and then leaves after telling you she has a boyfriend.
Kaname is the calm character that has no special features about him other than being tall.
Even a character like Momoharu who wasn't a bad character at all, was pretty much downgraded Hanamichi from Slam Dunk.
It's like almost all the characters are so one-dimensional that you are either X or Y.

Fourth, the romance. I could go on and on about the badly written romance but it would take a lot of time which isn't really worth it. For a small brief on it, read my description of Madoka.

I sadly gave a lot of time to this manga which made me really disappointed, I watched 29 eps of the anime and I went to read the manga to continue the experience only to be faced with a very big disappointment. I remember disliking the manga so much I was literally skimming through the chapters only to just drop it because of how bad it turned out to be.

Overall, I would give the manga a 3/10. Really disappointing manga for what could have been a classic.
Wouldn't recommend it to anyone including hardcore sports/basketball manga fans.