It's more or less an open secret, or not even a secret anymore but simply a fact of life, that Kieran Gibbs is firmly a backup for Arsenal at this point. Once considered a promising defender for both club and country, his development has hit a few snags while Nacho Monreal has soared to take over first choice duties at leftback.
While it would undoubtedly do Gibbs a massive service to go out and get solid starting minutes for a club looking to compete for a European spot, rejecting this move makes sense given that it would leave Arsenal one short in leftback depth. Additionally, Monreal might have to pull emergency centerback duties given the club's injury woes and the relatively young age of backups Calum Chambers and Rob Holding.
Gibbs, at this moment, might not be the player some hoped he would be but he's far from expendable and should have a solid role to play this season (barring injuries).