Powell had forgotten to nominate a penalty taker for the first-round tie but was spared any embarrassment as Pigott sealed the emphatic 4-0 win.
Simon Church scored twice on his home debut and Danny Green was also on target as a second-string Charlton side swept past Oxford.
Powell said: "There was still time for the lads to get some minutes because they've worked ever so hard in pre-season,"
"It was a chance to blood some of the younger players. Simon Church is still catching up and that was Joe Pigott's debut.
"So I felt we had the right balance for a game like this and it was a good experience. It was also a chance to rest one or two, so they have given me plenty of food for thought.
"There is a great feel-good factor around the place because the boys have performed. Oxford had their moments, but I feel we showed our ability to create and score goals."
United manager Chris Wilder still thought the 4-0 scoreline flattered the hosts.
Wilder said: "It was a little bit [harsh]. We were playing against a good side
with some Championship players. They [Charlton] freshened it up and they've got players out there who want to get in the side on Saturday.
"It was a good test for us and I'm delighted that we chucked a couple of our kids in. I thought they were first class and didn't disgrace themselves.
"People looking at the result will think we've had a mauling but as soon as we
get off the bus tonight we'll forget about it."