The midfielder went into theatre this week after rupturing the anterior cruciate ligament in his right joint during the Leeds United game.
Kieftenbeld faces up to nine months on the sidelines, but Manager Garry Monk said you can never really put an exact timescale on recovery.
"The operation went really well," Monk disclosed. "He's in good spirits.
"The key now is to do your rehab properly, and do it at a high level. He's got that mentality.
He will have all the support around him. We're happy it went well, the operation."
Monk continued: "In terms of timescales, it's too hard to tell. It depends.
"David Davis, he could have been a lot longer out than he actually was (after fracturing his ankle in pre-season). He did the rehab properly and came back earlier.
"It depends on the rehab and each stage of that. Right now it is about the operation going well, that's the first ticked box."
Monk said that Kieftenbeld's mental strength would help see him through.
"I spoke to him in the next day or two after it happened.
"I've been through a similar experience myself as a player.
"The advice you are given is that the sooner you get your head round it, accept that it has happened, and focus on what you have to do, the better for you.
"I have talked to him about the rehab, how you have to focus even when it gets hard and you become frustrated. In those moment you need to push through it.
"I can't see there being a problem with that."