Martinez proved himself after Leno suffered an injury towards the end of last season and was brilliant in goal as the Gunners won the FA Cup.
Arteta had a decision to make over choosing his number one and he decided to go with the German and Martinez was sold to Aston Villa in the summer transfer window.
The Argentine has been a top-notch goalkeeper for the Villans this season, but Leno has looked suspect in between the stick for the Gunners.
Sinclair stressed that based on last season’s performance, it was clear that Martinez was the better goalkeeper and the Arsenal manager made a big mistake in sticking with Leno as his number one.
He said on talkSPORT: “A huge call for me but I thought I had seen it last season when he was playing, Emi Martinez is better than Bernd Leno.
“I feel there have been a lot of mistakes made by Mikel Arteta.
“And in general I don’t see many positives going forward for them under Arteta.”
Leno has come under pressure after making some crucial errors in recent games, but Arteta has few quality options beyond the German in his squad at the moment.
