The Republic of Ireland international has been missing since making his first appearance of the season as a substitute in the EFL Cup second round meeting with Yeovil Town but is now working hard with the medical team at Finch Farm to get back to fitness.
“Darron has had a similar operation to James McCarthy," Koeman told evertontv. “He will be out for about three weeks. It always takes around three or four weeks to recover from that surgery but he's doing well."
The midfielder signed a new two-year deal in the summer and enjoyed a successful pre-season under new boss Koeman, featuring in four of Everton's six friendlies.
“It's always frustrating for footballers to get an injury and to need an operation," Koeman added. “Then you lose time and training and the league is only just starting. That is always difficult but it's not a long-term injury and hopefully Darron, and James, will be back in the team for training as soon as possible."