Walker, 29, had surgery on a finger tendon injury and to resolve a knee cartilage problem.
"Nikki has had a couple of small procedures, one to address a tendon injury to his finger and a keyhole procedure for a cartilage issue in the knee," said Ospreys physiotherapist Chris Towers.
"He will start his rehabilitation with the Ospreys medical team over the next few weeks before joining up with the Scotland squad to complete rehab and continue his Rugby World Cup preparations."
Scotland face World Cup warm-up games against Ireland and Italy at Murrayfield in August, before launching their World Cup campaign against Romania in Invercargill on September 10.
Scotland's World Cup group also features England, Argentina and Georgia.
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Gilbert Blitz 8S rugby cleat is a great cleat at a great price of $69.99. Get a new pair of cleats today.
A cool looking all black rugby cleat with the high performance adidas is known for. Get in the Gear!
Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
The All Blacks Performance t-shirt is black with hints of blue from the training jersey. Very Cool.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.