Monreal has been linked to LaLiga side Real Sociedad, who have reportedly agreed a two-year contract with the Spanish full-back.
The 33-year-old, who joined Arsenal from Malaga in 2013, started Saturday's 3-1 Premier League loss to Liverpool.
Amid speculation over Monreal with the LaLiga window set to close on September 2, Emery told reporters: "He made a very big match and he's a very big professional for us.
"This is one issue we need to speak in the next few days because in Europe they don't finish the transfer window yet, but we are going to speak about all the situations and the possibilities. It's the reason I was speaking about it not being normal that we finish two weeks ago in Europe because the squad is not completely closed for us at the moment.
"Some players can leave but we cannot sign another and it depends a little over the next days how we can decide about some circumstances with some players. With Nacho one possibility is open and we are going to speak about that."
Emery was speaking after Arsenal succumbed to Joel Matip's opener and Mohamed Salah's second-half brace at Anfield.
Lucas Torreira scored a late consolation for Arsenal, who had their 100 per cent start to the season ended by table-topping Liverpool on Merseyside.
"Really, I'm disappointed with the result. It's the first analysis we can do but after 90 minutes we did different situations with the players, tactically and spiritually," Emery said.
"I think the team showed in one moment good character and it's one step ahead we want to do. The result means we're not happy today, but we can be optimistic for another situation."