Bournemouth struck twice in the final minutes to win 2-1 at Portman Road.
McKenna said: "It is an extremely disappointed dressing room. I thought it was a good performance with some outstanding performances.
"Conor Chaplin with his goal, he's on a great journey. We know we could have won the game. I think the second goal being ruled out was harsh.
"Conceding late changes the momentum and then they go and grab a second."
He also stated: "The game was in such a good place we normally make subs earlier but we were doing well. We weren't giving away too much and we were getting stronger but it's a poor moment on the first goal and then you're vulnerable. They had five fresh subs on the pitch and momentum.
"It's a big blow but the group is working so well. We've had a disappointing week but we are working hard and competing with teams.
"We deserved more from the game today but it's a disappointing week. The margins are really small for us getting that win and the jump to getting more points isn't a huge one."