Martin Guptill and Tom Latham's rollicking 118-run opening stand on Monday in just 8.2 overs saw to that.
"I guess that is one of the downsides of the two heavyweights [Guptill and Brendon McCullum]. Some guys haven't had a hit in the middle," McMillan said.
"A lot of them are dying to have a bat, but when you sit back and see what the top order are doing it's pretty exciting stuff. I guess it is perhaps a small concern. The key is to make sure they've prepared well and are ready to go."
The goalposts move for two New Zealand players today.
George Worker, unwanted in the first two matches, heads back to Central Districts in their second round Ford Trophy game against Wellington at the Basin Reserve; while Matt Henry, a star turn with the ball, returns to Canterbury and their game against Otago at Alexandra.
The idea with Worker is to give him some batting time to keep him in work; while Henry was always only picked for the first two games. Tim Southee returns to the squad after having two matches off.
You can't accuse Henry of not grabbing his chance. He took four wickets in each of the two lacklustre Sri Lankan innings - 8 for 82 off 19.4 overs - bowling with accuracy and pace to spearhead impressive group bowling performances.
Henry didn't quite measure up on the tour of Australia, but there's confidence he will be on song if required during the Sri Lankan series.
McMillan is optimistic Williamson will return to the side tomorrow, providing he gets through training drills today.
It will be a day of assessing McCullum's disc injury, suffered crashing into an advertising hoarding in the field on Monday.
It's been an accident waiting to happen for a few seasons, given McCullum's full on approach to chasing balls to the boundary rope.
"There's no off button," McMillan said of the skipper's fielding philosophy.