11 in midlane isn't that bad at your ELO, but it's not godlike and maybe your midlaner plays against good midlaners (15 or 17 in midlane at your elo).
Plus maybe your strategy on your midlaner is bad, poor positioning in lane and teamfight, poor share between dmg/tankiness/utility, a championpool consisting in late game champions that don't reach their full potential before the end of the game, maybe your player is exhausted or unexperimented because he doesn't play soloQ.
Don't focus on that player since you cannot improve his stats because he is old, you can improve little things i mentioned earlier but try to find another titular midlaner.