Roy practically loses his marbles in the novel, so maybe that's where all this imagery of loose spherical objects comes from. You might start to think you need to go to the eye doctor as you read and see all these spots, but don't worry, it's just our baseball-crazy narrator talking circles again.
The most obvious example of the spherical imagery is the baseballs that show up in pretty much every chapter. The game may be played on a diamond, but it's all about the ball. The first time it shows up is in the very first paragraphs, when Roy thinks he sees some kids playing catch from the train window:
As he was looking, there flowed along this bone-white farmhouse with sagging skeletal porch, alone in untold miles of moonlight, and before it this white-faced, long-boned boy whipped with train-whistle yowl a glowing ball to someone under a dark oak, who shot it back without thought, and the kid once more wound and returned. (1.2)
Roy doesn't know whether or not this appearance is real or fantasy, but check out the way that the scene is set: "bone-white," "skeletal," "long-boned." It's all pretty deathly, which gives us a sense that, even though Roy loves the game, there are some ominous consequences that come along with baseball.
The sinister associations with spheres pop up again when Roy sees Gus in the night club, and starts losing bets with him. Gus has a glass eye:
[…] Roy found himself staring into a pair of strange eyes, a mournful blue one and the other glowing weirdly golden. His scalp prickled as the bookie, a long stretch of bone, rose to his full height. (4.150)
Later, Memo will tell Roy to go see a fortune-teller, who looks into a crystal ball to tell his future. However, as she's looking into the crystal ball, "a blank expression came over her face and she slowly shook her head. [. . . ] 'The future has closed down on me'" (6.60-62). Even though the fortune-teller's crystal ball was handing out good news (he'll fall in love with a dark-haired woman) something ominous is going on.
We finally get the skinny on the circles when Roy starts to hate the game:
Sometimes as he watched the ball soar, it seemed to him all circles, and he was mystified at his devotion to hacking at it, for he had never really liked the sight of a circle. They got you nowhere but back to the place you were to begin with. (8.15)
This really gets at the heart of Roy's problem. He wants to move on but always finds himself trapped and frustrated; that's what the circle's about. Instead of getting ahead, moving up in the league, becoming a champion, and retiring rich, Roy just keeps finding himself back where he started: injured, impoverished, and frustrated. The baseball, that never-ending circle, is the perfect image of Roy's spinning his wheels and never getting anywhere. He likes the train and he likes to keep moving, he tells us, because it keeps him away from those circular thoughts in his head.