Iwelumo's strike, a spectacular volley on the turn that flew into the top corner after 74 minutes, was the only highlight of an otherwise drab game but Johnson was pleased with the performance.
"It's not just the bouncing back that's delighted me," he said. "It's the nature of the performance as well.
"We felt we had to win all our battles over the field and then try and play our football and the tactic worked very well for us on the day.
"Our club is not bad at coming back from adversity.
"We have a good group of honest lads and it hurts them when they lose a game and today they proved how much it hurt them on Saturday."
Johnson reserved special praise for Scotland international Iwelumo after the on-loan Wolves striker's matchwinning moment.
"I had just called Chris over and told him I was bringing him off soon and that he needed a bit of magic in the next five minutes," he said.
"He said 'alright' and went and produced it but that's Premier League players for you."
It was Paul Hart's first defeat as Palace manager in only his second game in charge and he thought his players looked tired following Saturday's win over Sheffield United.
"I thought as good as were on Saturday, we were equally as bad tonight," he said.
"We looked a bit weary, which is understandable with what the players have had to put up with and what they have gone through.
"But you do know they have got it well within them to bounce back.
"We have to recover properly, rest and get ready for the next one.
"I have just said to them in the dressing room, 'look this is us, there's no more coming in and there is definitely no-one going so it is me and you and that's it, that's what we have got'.
"I said we have got seven weeks and there's three that go down, so if you finish fourth bottom you have achieved.
"In mid-season when you get thrown with a points deduction like that it is a real kick on the nether regions. They have had to cope with it and it's not always easy - even for strong players like ours."