Kolasinac remains out until October, but the positive news for Arsenal as far as the left-back position goes is that Monreal has now resumed full training.
The versatile midfielder, who was playing at left-back on Sunday, hurt his leg in a challenge with Kyle Walker and despite trying to play on, was eventually substituted for Stephan Lichtsteiner after 35 minutes.
This is a blow for the Gunners, especially after only one game played so far this season.
The Gunners head to Chelsea on Saturday, and provided an injury update on Thursday morning, ahead of Emery's scheduled press conference.
Maitland-Niles limped off the pitch in the first-half of Arsenal's 2-0 Premier League opening day defeat to the reigning champions and now doctors have confirmed that he has suffered a "small fracture" to his left calf bone.More news: Djokovic Wins In Cincinnati
More news: Cardiff City vs Newcastle preview: It's Benitez vs Warnock
More news: FBI fires Strzok in wake of anti-Trump text messages
Defender Carl Jenkinson has also suffered a sprain on his right ankle and will be out for upto two months.
The injury to Maitland-Niles was obvious but now, the club has confirmed the extent of the knock picked up by the 20-year-old.
The former is likely to return to training from his knee problem in October, while Koscielny is continuing his recovery from Achilles surgery which cost him his place in the World Cup finals squad and ultimately a winners medal.
Monreal has made over 200 appearances for Arsenal since joining the club from Malaga in 2013.
Arsenal's next Premier League fixture is against Chelsea this weekend.