Coquelin transfer: Arsenal’s mistake?

Arsenal’s preparation suffered a big setback because they lost a dear player, Francis Coquelin. 

He is out on loan at Charlton Atletic for at least four week. Coquelin is a volatile player, capable as leftback and right back. 
He was much of a success at the recent 2-1 League Cup defeat to Southampton. However, there are others such as Calum Chambers and Hector Bellerin.
Although he has not been able to play in the defence but he has been able to satisfy curiosity of his masters.

What this means is that Wenger will have fewer options of players to call on in the following few matches.

That brings up the questions about how logical it was to send him on loan in the first place.