The Town boss made six changes to the side that got their Championship campaign under way at Reading at the weekend, handing debuts to Frederic Veseli, Christophe Berra and Dean Gerken as well as giving Paul Taylor his first start in 11 months.
After surviving the first half at the Lamex Stadium, Ipswich succumbed to second-half goals from Felipe Morais and Jordan Burrow as the League One side progressed to the second round.
"I thought the first half was alright, to be honest," said McCarthy.
"We expected it to be a difficult game but we looked alright in the first half and then in the second half we give a penalty away that the lads are not too happy about, but until I see it I don't know.
"Then when I changed the team and put two lads on we had a bit of a resurgence but we gave another goal away that killed us off.
"I've no regrets whatsoever about the amount of changes that I made because I doubt I could play our two centre forwards, Daryl Murphy and David McGoldrick, in a game of that intensity week in, week out.
"The other two have to play and Dean Gerken has had not much to do apart from pick the ball out of the net twice so I've no regrets at all - they're all first-team players."