The New Zealand captain is out of tomorrow's fifth and final ODI against Sri Lanka at Bay Oval and was not in the T20 squad for the next two matches anyway.
McCullum damaged his back crashing into advertising hoardings at Christchurch during the second ODI on December 28. There had been slight hopes he might have been ready for the fourth match at Nelson on Saturday, which only lasted nine overs.
But now he will rest until the start of the ODI series against Pakistan.
"He's really struggling," New Zealand coach Mike Hesson said today.
"He hasn't progressed as well as we'd thought. He will get a further assessment.
"Obviously he wasn't playing the T20s so he'll have a decent break before Pakistan arrive and certainly before Australia arrive."
The schedule has Pakistan starting their tour with three T20s, to follow straight on from Sri Lanka's two 20-over games on Thursday at Mt Maunganui and at Eden Park next Sunday.
That will give McCullum another 21 days to recover before the first ODI against Pakistan at the Basin Reserve on January 25.
Australia's highly-anticipated return trip starts with three ODIs, beginning at Eden Park on February 3.
That tour becomes the main focus for McCullum, who is retiring after the second test against the Aussies in Christchurch, starting February 20.
New Zealand management hope to know more tomorrow about fast bowler Tim Southee's left foot injury, which struck in Nelson early in his bowling stint in the third game at Nelson last Thursday.
Southee suffered the injury in his first over and struggled from that point.
He won't play tomorrow and is unlikely to feature during the two Sri Lankan T20s which round out this week.
