It was proper. If it was on the players left shoulder it would look ‘normal’. When it is on the players right shoulder it would look ‘backward’ but that is correct. It is ‘backward’ so that the blue part faces the direction you are walking. This is true on regular soldier uniforms as well.
Think of it like a flag on the back of a motorcycle blowing in the wind. No matter which side you look at the moving motorcycle, the blue part (union) is always facing forward. :)
Currently, the flag is worn on the right shoulder and the field of stars faces forward. The reason the team had the flag reversed is because that’s how the 10th MTN wore the flag in WW2. Also, the flag the players wore on Saturday only had 48 stars, because the US didn’t have 50 States when the flag was worn. Beat Navy!