Boro were on top in the first half at The Den but a change and tactical tweak from Alex Neil at half-time swung the game, before the Millwall boss then made a treble change just before the hour mark.
The Lions then scored the only goal of the game 20 minutes into the second half, before Carrick had turned to his bench.
The situation was similar back in February when Boro lost at Bristol City.
With Boro leading 1-0, Robins boss Liam Manning made a treble change early in the second half and George Earthy, one of the players introduced, hit two goals before Carrick made a change as Bristol City fought back and secured what could turn out to be one of the decisive results in the race for the top six.
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In three of the last four games, Carrick’s first unenforced change has come in the 66th minute – but the head coach says the clock is “irrelevant” when judging substitutions.
He said: “You make your subs based on the feel of the game.
“The game the other day, for instance, we started making plans five or six minutes before we make the change – getting the boys ready etc.
“We had an idea in mind in how we wanted those boys [McGree and Forss] to come on and make a difference.
“It just so happened that they scored just before we were ready to make that change. The clock, in many ways, is irrelevant.”
Asked how he weighs up what substitutions to make and when, Carrick said: “How long have we got? It always depends.
“Sometimes it’s quite straightforward, sometimes it’s quite obvious, and other times it might be certain spaces or ways we want to attack, certain runs we want to make, certain attributes we want to get into the pitch to influence the game in our favour in whatever way.
“Most of the time, we make the change with how we can make a difference in a game positively. The odd time, we’ve maybe put an extra midfielder and defender on when we’re seeing a game out late on.
“There’s all sorts of different factors: players attributes, who’s feeling it, looking tired and who’s looking strong.
“There are all sorts of factors – physical, mental, tactical, managing the game in the stadium.”
Carrick says he also tries to strike the right balance in making changes to counteract what the opposition might be doing while also trying to play to Boro’s strengths.
He said: “Picking your starting line-up is the exact same challenge. Certain teams you feel like you have a good idea of what it’s going to look like, but everyone has a right to change and do something different. That’s part of it.
“Some teams change a lot, which, in some ways, makes it harder to predict, but it’s equally new for themselves every week as well.
“We choose to be pretty consistent and we feel that it gives us the benefit of knowing who we are, where we are and where we should be in terms of positions and that kind of thing.
“That will mean other teams feel they have an idea of what we’re going to be like, and we understand that. There are so many ways of doing it.
“There is a lot that comes down to individuals as well. That’s exactly why I always talk about the importance of variety.
“We have players who play the same position but in very different ways. It’s the constant tinkering and decisions that we try and make the best results, while appreciating everyone is trying to do the exact same thing.”
