In the patch notes for 9.13, Sivir received a massive nerf which not many people noticed.
"MANA REGEN 8.012 ⇒ 8"
Now, since mana regen is calculated per 5 seconds, meaning that Sivir will now regen 8 mana per 5 seconds instead of 8.012, every 416.67 seconds, or roughly 7 minutes, Sivir will be 1 mana point short.
According to LeagueOfGraphs, the average player is ranked silver in Solo/Duo. The average game duration in Silver is 29:44 Minutes, Meaning that throughout the duration of the match, Sivir will lose 4.281 mana points compared to previous patches.
As a Garen main, I do not know if that's a lot of mana or not, but still this seems uncalled for.