I think a few changes to the characters and use the baseball games as a way to show progress and it could definitely be serialized.
The character's cores would remain the same, but instead of the MC being lame for the sake of it (which works for a one shot) he would need a reason for it. Like what if he was trying to process the death of his mother, and his friends ditch him because of the negative behavior he exhibits be cause of it. This would kind of scar him. In the moment he wouldn't understand but it would be his depressed state and inability to think positively.
Main Girl would have a bit of on obsession with baseball. Not enough to color her personality and change who she is but enough to know what she's looking for. She would come to know the MC from his notoriety in middle school. He was a great player but declined really fast as he had to deal with the death of his mother and ultimately quitting.
She would then find him roaming the halls of their high school and take notice. This would lead her to take the actions she does in the one shot.
He would still get some strong feelings for the other girl, and the beginning would play out like it did only he'd join the baseball team late.
It think that does it for serialization fixes.
So I'm wrong as others have pointed out. Noted.
I'd also rather not be pinged by this again please and thank you.