McCullum's words were a rare admission from this England camp that they have made an error. They usually remain steadfast in their wish to look forward rather than back and find the positives in any situation.
Four of England's highest 10 fourth-innings chases have come under Stokes and McCullum, including 371 to win the first Test last week and 378 against India at Edgbaston in 2022.
That has influenced Stokes' decision to bowl first in 10 of the 11 matches he has won the toss in England. This is only their second defeat when doing so.
"We thought this pitch might get better to bat on as we went through the five days but as we saw it didn't," McCullum said.
Rather than blame the toss, Stokes pointed to England's inability to knock over India's lower order in the first innings, where they recovered from 211-5.
"No-one's got a crystal ball, no-one really knows what a wicket's going to do," Stokes said.
"Having them 211-5, we were happy there - even five down at the end of day one.
"We just weren't able to bust them open. We had a chance to potentially bowl them out for under 350 on that wicket, which would have been a good effort."
Stokes said he was not concerned by his bowlers' toil during the match. England have responded by adding seamer Gus Atkinson, who has missed the first two Tests with a hamstring injury, to an otherwise unchanged squad for the third Test at Lord's which starts on Thursday.
Jofra Archer might also come in for his first Test in four years after a series of injuries, while Sam Cook and Jamie Overton could also come into contention given the quick turnaround.
England squad for third Test against India: Ben Stokes (captain), Gus Atkinson, Jacob Bethell, Ben Duckett, Zak Crawley, Ollie Pope, Joe Root, Harry Brook, Jamie Smith (wicketkeeper), Chris Woakes, Brydon Carse, Josh Tongue, Jofra Archer, Sam Cook, Jamie Overton, Shoaib Bashir.