The United boss revealed the calf problem, that has kept the midfielder sidelined in recent weeks, is much worse than first thought.
Van Gaal said: "Carrick is not available, it was more heavy than we thought. We tried everything but now we have to think about next season.
"He shall not play anymore this season, it's better now to focus for him on next season and being fit to start."
And the Dutchman is hopeful he will make a quick recovery as the statistics clearly show his positive impact on the club's performances.
He added: "When you see the stats, it's clear that he's very important but that's also because we have only right-sided holding midfielder.
"That's what I am saying already in the whole of the season, that our selection is not balanced, and I want to restore that in the next transfer period.
"The life is not ending at 34, but for a professional football player, it's close by. That's why I have to handle again in that position."