England may drop Chelsea's Terry
McClaren's worry is not over the state of Terry's fitness, more the yellow card he carries into the game.
Together with Rio Ferdinand and Ashley Cole, the England captain is one caution away from missing the trip to Russia on October 17, which is why McClaren is pondering a move which Terry is unlikely to be in full agreement with.
"It is a consideration," confirmed McClaren. "We have to look at the bigger picture as well as the smaller one.
"It means picking a team for Estonia is not an entirely straightforward issue, there are other factors involved as well.
"Clearly though, our priority is to win and avoid dropping points at home as we did against Macedonia. If we do that, what happens in Russia is irrelevant."
Even if Terry successfully argues his case for inclusion against Estonia, concern still persists over the lengths he is going to in order to pursue his career.
In the build-up to last month's double-header with Israel and Russia, the 26-year-old claimed spending later life in a wheelchair would be preferable to missing important games just now.
While the comment can hardly be taken seriously, McClaren insists no medical staff would allow him to put his body at risk.
"John will know where the line is between being brave and foolhardy and the medical people will know it too because they are the ones who are closest to him," he said.
"I picked him as captain because he epitomises what an England captain should be.
"On Saturday night, when everyone was saying he would be out for weeks, he told me he would be playing on Wednesday.
"He wants to play in every game and miss nothing. But it is a difficult balance. You would never put any player at risk."