There are always those players that a team signs for whom one has very high hopes. A player who, for whatever reason, was either struggling at his current club but clearly had talent, or was very good and wanted to progress his career by signing for a bigger club.
Mathieu Debuchy was the latter. He had already displaced Bacary Sagna in the French national team at the 2014 World Cup, and was signed to do the same at Arsenal after Sagna left for Manchester City. Things didn't really work out the way Debuchy wanted, though, as he quickly fell out of favor at Arsenal.
Last year was injury-wracked; he spent three months out with ankle problems, then came back, played for a couple weeks, then got Stoked in the shoulder, which kept him out for three more months. He played a bit in the FA Cup in 2014, but not in the final; after deciding this summer to stay and fight for his place this year, he has made only seven appearances for the club and is not really in Wenger's plans, particularly with the emergence this year of Hector Bellerin.
As you would expect, this doesn't make Debuchy happy; at 30, he wants to go to a place where he can play regularly, and it doesn't seem like Arsenal is that place. I wish him no ill; I was excited for the signing when it happened, and I'm disappointed it didn't work out. There's interest from Aston Villa and Bayer Leverkusen, and Debuchy would also be OK with a loan move somewhere, but it would appear that his time at the Emirates is coming to a close.