If Karasuno had lost to Nekoma in their first practice match in the Haikyuu!! series, it would have been a significant setback for Karasuno's team morale and confidence. The loss could have led to self-doubt among the players and potentially affected their performance in future matches. Additionally, losing to Nekoma early on could have altered the dynamic between the two teams, potentially changing the rivalry and interactions between Karasuno and Nekoma throughout the series.