Unai Emery expects Petr Cech to be sidelined for up to three weeks after the Arsenal goalkeeper limped off during Saturday's 2-0 win over Watford.
Cech suffered a hamstring injury in the first half at the Emirates Stadium and Arsenal manager Emery fears the former Chelsea star will be sidelined for several weeks.
The 36-year-old will be assessed by Arsenal's medical team on Sunday.
"I think it's a hamstring. Maybe he will miss two or three weeks. I hope not more. We will see tomorrow," Emery said.
If Cech's injury absence is confirmed, Emery can at least rely on Bernd Leno to prove an able deputy judging by the German's impressive display against Watford.
With Arsenal under the cosh in the second half, Leno made a fine save to deny Troy Deeney and coped well under the Watford bombardment.
It was Leno's first taste of the Premier League after his only other appearances came in the Europa League and League Cup.
Emery called on the pre-season signing from Bayer Leverkusen to seize his chance, saying: "I'm very happy with Leno.
"I think he was content to wait for this moment and now it is arriving. His performance was the best for us today."
Arsenal's fifth successive Premier League win, and seventh in all competitions, was harsh on Watford, who bossed the game for long spells before Craig Cathcart's 81st minute own goal.
Mesut Ozil's strike two minutes later put a flattering gloss on the scoreline for Arsenal, who are up to fifth place.
Emery conceded Arsenal had laboured at times, but he was pleased with the way they kept fighting.
"We knew it would be a difficult match. The ball was near our box a lot. The rhythm of the match was not what we wanted," he said.
"In the first half we were a little nervous. We had to improve with our positioning, then our moments were arriving and we found our efficiency to score.
"We need players being hungry, to come in and help the team. Today we had Leno, who came on for Cech, then also Danny Welbeck and Alex Iwobi. They showed their quality.
"Each match is very important to win to help our confidence.
"We need the whole 90 minutes to have this control. We need more control, with the possession and the positioning."
- Frustrated Ramsey -
Emery also needs to resolve the tricky issue of Aaron Ramsey's future.
Ramsey is out of contract at the end of the season and negotiations over a new deal are reported to have broken down.
The Wales midfielder, who has attracted interest from Chelsea and Manchester United, was a subdued figure against Watford before being hauled off early in the second half.
Ramsey responded with a shake of his head and appeared to ignore Emery when he took his seat on the bench.
Emery wasn't fazed, saying: "When the player wants to play every moment, I want these characters. They can help the team."
Watford have now gone three league games without a win, spoiling their flying start to the season.
But Hornets boss Javi Gracia rightly felt his side had been unlucky not to emerge with at least a draw.
"What can I say? I'm very upset for my players. They deserved a better result," he said.
"We had a very good performance, with a lot of clear chances to score. Arsenal had some amazing saves and if you don't score you can lose.
"It's frustrating. In the last few games we feel we deserved more."
