"I will not be ready to take charge of Wales for the international against the Barbarians and if I recover enough to get to Australia during June I am still unlikely to be mobile enough to play a full coaching role.
"It has been difficult to accept that advice, but it just makes me more determined than ever to get back into the driving seat with Wales later on this summer as we prepare for an Autumn series which includes games against New Zealand and Australia."
Howley was one of Gatland's first appointments when he took charge of the Welsh coaching set-up in January 2008.
He won 59 caps for Wales and captained his country on 22 occasions.
Speaking on his new appointment, Howley said: "I will give 100 per cent of my effort to ensuring we are fully prepared to meet the challenges and take the great opportunities which lie ahead in this summer series.
"We will be working to secure victory against the Barbarians after the defeat we suffered last year and the Tests in Australia are an opportunity for us to deliver a win in the southern hemisphere.
"Along with the rest of the coaching team I am immensely proud to have been offered this opportunity and we will give our all until Warren is ready to take full control again."