Allardyce made nine changes to his side for Sunday's trip to the City Ground, and his young team was brutally exposed as Championship outfit Forest cruised to an emphatic win.
Jamie Paterson was the star for the hosts, the striker scoring a hat-trick, while Djamel Abdoun and Andy Reid also found the net, leaving Allardyce to lament a dismal West Ham performance.
West Ham face Manchester City in the first leg of their League Cup semi-final on Wednesday, and Allardyce believes he had to treat that clash as his priority.
"I don't think I had a choice," Allardyce told ITV.
"I'm the manager, I make the decisions for the benefit of everybody at the football club.
"We go to Manchester City on Wednesday night. Manchester City played yesterday, we had to play today.
"We had a huge amount of problems in terms of the available members of the squad so I've got to keep them intact, that's a major priority.
"Everybody changes their team in the Capital One Cup early on in the season but doesn't make a fuss about it.
"Many, many managers have changed the team in all divisions, but my changes had to be severe, I didn't want it to be but it had to be."
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