Milligan was visibly upset with Arnold's decision to substitute him midway through the second-half. Though showing no serious dissent, Milligan gestured confusion over the change.
Arnold wasn't pressed on Milligan's reaction after the game, but warned of changes ahead of their next group tie against Argentina.
"The adrenaline will kick in for sure when they play Argentina in a couple of days," Arnold said.
"Tomorrow will be about recovery and a light training session, and we're ready to go again - that's the beauty of this competition.
"It's the same for all teams and our players are as fit as they have been probably in their careers.
"I probably will make a couple of changes because the players put a lot of energy and effort into tonight.
"We've got 18 players."