In five tests against Australia, Southee took nine wickets at an expensive 62 runs apiece and was far from the cutting edge New Zealand had hoped for.
New ball partner Trent Boult also had issues against the Australians, but still snared 18 wickets at 42.1.
Injuries to his back and foot lessened Southee's effectiveness this season. McCullum, who defends Southee's contributions as a senior member of the leadership group, admitted New Zealand's lack of penetration with their seam bowlers was a key point in the two series.
''There are other guys coming through who are performing very well, Matt Henry in particular, and it's going to push Tim to make sure he's operating at his best, which we saw for a good 12-18 months," McCullum said.
''I still believe he's a strong leader in the group and he's just got to smooth out some of his rough edges."
McCullum laid off offering an opinion on whether Southee might need to cut back on his international commitments across all three forms of the game. That, he said, was for Southee to talk about.
Boult and Southee are currently ranked No 8 and 9 in the world. Boult was No 2 in July last year; Southee got to No 5 in June 2014.
At the same time, McCullum gave spirited left-armer Neil Wagner a huge rap for his performance in the second test loss to Australia at Hagley Oval.
Wagner took seven wickets, turning in a herculean effort of persistent short-pitched bowling at high intensity for a sustained period, following a set plan, and yesterday with a fracture in his left knuckle.
''It takes a toll on any bowler to bowl two or three overs of shorter-pitched bowling," McCullum said.
''For him to be able to do it for 35 overs of a test is phenomenal.
''He's going to be a huge player for New Zealand over the next couple of years because he's got that little bit of mongrel about him, that he just wants to get the job done."
McCullum conceded Wagner isn't the most skilful cricketer in New Zealand ''but geez it's hard to go past him in terms of his attitude."
