Arsenal boss Arsene Wenger admits Shkodran Mustafi and Lucas Perez were "a bit surprised" by the pace and intensity of the English Premier League.
The two made their respective debut for the Gunners against Southampton with the German playing the full 90 minutes and the striker replaced after 62 minutes.
Mustafi is expected to start on Tuesday against Paris Saint-Germain but the Lucas will be on the bench with Olivier Giroud leading the Arsenal offensive department.
"I feel both are short [of fitness] a bit, they lack competition and their second half was much better than their first half," Wenger told reporters.
"They were a bit surprised by the pace and the commitment of the Premier League [but] they will adapt to that. As well, they were a bit on their nerves in the first half. In the second it was much better.
"He [Lucas Perez] has had one game in La Liga, and then he didn't play many more because they knew he would go. [Shkodran] Mustafi has only played one half since the start of the season with Valencia, so they are missing games.
"I have to decide that [if they will feature on Tuesday].
"We will see how everyone is tomorrow morning. For Tuesday, we might just have Gibbs coming back, who was not in the squad today. It's possible they are in the squad."