The fifth round tie on Monday night finished 1-1 after extra-time and went to penalties, where Forest won the penalty shootout 5-4 with Matz Sels saving from Jack Taylor.
Ipswich boss McKenna said afterwards: "It is a disappointing way to lose. Great effort from the players. Really challenging night in so many aspects and really proud of the effort.
"I thought we took pretty good penalties to be honest. They took great penalties as well. It came down to a tiny margin."
On taking positives from tonight's game into the league, he added: "Of course. A team with a lot of changes, a team that go through the background of where the team has been and how they have grown.
"We met a lot of challenges tonight and we met them head on. We gave a performance to be proud of it just didn't go our way."