Barkley, who only just returned to training after recovering from groin surgery, suffered the setback on Monday (14 August) and now faces a prolonged spell on the sidelines. The 23-year-old needs to undergo further medical tests in order to discover the true extent of the injury, but he is not the only injury concern Koeman is having to deal with.
Spanish forward Sandro Ramirez is a doubt after suffering a knock during Everton's victory over Stoke City on Saturday, while injury-prone midfielder James McCarthy sustained a knee problem during training. With four games in the next 11 days, Koeman's resources are set to be stretched.
"We have some problems," Koeman said in his press conference. " [James] McCarthy's out with a knock on his knee, Sandro Ramirez is doubtful. We will make a decision tomorrow morning."
"[Barkley] stopped training last Monday, it looks like a hamstring injury but we need to do more tests today and tomorrow. We have 17 outfield players, it's enough for tomorrow but we need more as we have a tough 11 days coming."
It was suggested earlier this week that Barkley may face up to six weeks out due to his latest injury, which may have an effect on his protracted move away from Everton. Tottenham's interest has been well documented in recent months but Mauricio Pochettino's men are reluctant to meet Everton's asking price, believed to be around £35m.
Premier League champions Chelsea are also believed to be interested in Barkley but their pursuit of Leicester City's Danny Drinkwater suggests they are currently looking at other targets. Tottenham are renowned for conducting their transfer business late in the window, and Koeman is prepared to wait until the end of August in order to see where Barkley's future lies.
"I don't know [if the injury will have an impact on a move]," the Everton boss said. "No, there's no update about his situation. I will wait until the end of August."
Barkley has one year left on his current contract at Everton, who have already brought in a number of attacking midfielders this summer. Davy Klaassen and Wayne Rooney have both arrived on Merseyside, while Swansea City playmaker Gylfi Sigurdsson is set to be announced as Everton's record signing within the next 24 hours. The former Tottenham midfielder underwent a successful medical on Wednesday morning and will join Koeman's men in a deal that could be worth £45m.