The Brazil midfielder, who joined Arsenal from Newcastle United this month in a deal reportedly worth 75 million pounds, needed ice on his leg after being substituted.
Mikel Arteta said the injury was improving but remained cautious over the 28-year-old's availability.
"It's evolving well, but let's see," Arteta told reporters.
Keeping his squad fit will be a major priority for Arteta as Arsenal begin their defence of the Premier League title. Injuries affected their campaign last season, with key players including Kai Havertz, Martin Odegaard and Gabriel Jesus spending significant periods on the sidelines.
Arteta believes Arsenal need to improve their player availability if they are to become even stronger.
"The more dominant we are, the bigger the margins, the better," he said. "We need to improve the availability of the squad in certain areas in order for those margins to become much bigger."
Arsenal have never successfully defended their Premier League title, despite winning three consecutive English championships in the 1930s.
However, Arteta believes his team are approaching the new season with a different mindset.
"I feel good. Different. Hopefully better. More excited than ever, hungrier than ever," he said.
"I have no question that our players have the desire. Now it's something we have to do daily."
Arsenal are also looking to strengthen their defence, with reports suggesting they have agreed a deal worth 51 million pounds with Aston Villa for England centre-back Ezri Konsa.
Arteta refused to confirm the transfer.
"When we're ready to announce something, we'll let you know," he said.
The Arsenal manager was more decisive when asked about Myles Lewis-Skelly. Despite speculation surrounding the 19-year-old's future, Arteta said he was "100% sure" the versatile youngster would stay at the club.
Other players, however, could face uncertain futures as Arsenal look to reshape their squad.
Gabriel Martinelli, Gabriel Jesus and Ethan Nwaneri have all been linked with moves away, and Arteta admitted that difficult decisions may be necessary.
"It's a really, really tough moment, but you have to face it with a real honesty and transparency," he said.
"We all know everything has a start and an end. When it ends, our job and duty is to do it in the best possible way, with respect and admiration for the player and the relationship.
"But at the end, we need to make difficult decisions."
