What are you even talking about? Alonso is a very good footballer, he was amazing for us last year and continues to provide the goods this term, even, if rightly so, he is a little bit burned out. The man is working his socks off every single game for the last year and can't even get the sheer acknowledgment for that from our fans.
I have no idea what are your expactations from a left wing back but all in all I can assure you there are not many better than this guy. I am really not sure if your beloved Alex Sandro is better than him this season. Anyway, you don't need a world class player in every position and I think our LWB position is the least of our problems.
Alonso needs rest and needs competition, but he is a good player and gets no recognition for his great service at this club, and really I'm quite tired of seeing people bashing him after every game (like really, what wrong did he do yesterday? he was slow?). Some of you here even try to take the credit off his goals because he is a wing back (?!)... Somehow only Chelsea fans see all these wrongs...