On a weekend where United number one David De Gea was at fault for two goals in the 4-0 humbling at Brentford, Henderson was a star player for Forest as they won their first home game back in the Premier League for 23 years.
United loanee Henderson said in an explosive pre-season interview that he was “fuming” at the way he was treated at Old Trafford, claiming he had been promised the number one spot following his return from a loan at Sheffield United.
And the 25-year-old backed up his words against the Hammers by saving Declan Rice’s penalty and producing several other key stops to ensure a clean sheet.
Boss Cooper did not want to get involved in the Old Trafford politics, but was impressed with how his goalkeeper played.
“I’m not interested in that, in terms of his performance he was excellent, he looked a really good goalkeeper, the obvious thing is the penalty save and the other saves he made,” he said.
“His distribution and decision-making was good. I thought tactically he was good with his positioning. He did well, he did well last week, did well this week and we need him to do well next week.
“At times you need goalkeepers to have big moments, like you need strikers to have big moments.
“Dean had some big moments but that is his job, he did it well. He will review the game properly with the goalkeeping staff and working even harder to keep improving.”
Had it not been for Henderson’s heroics, the Hammers, who also hit the crossbar twice, would have got their season up and running.
Instead boss David Moyes was left to reflect on successive defeats and a feeling of his side not quite being up to the level yet.
They begin their European adventure in the Europa Conference League play-off first leg against Danish side Viborg and Moyes does not see it as a distraction.
“It’s an absolutely fantastic thing. We finished seventh in the league last year and you’re talking about is Europe good?” he said.
“My goodness, I don’t know how many teams in the league would shake your hand and say, ‘Please give us European football’. We have a play-off and we want to try and get in the group. It’s a great thing for West Ham.”
Meanwhile, Joe Lolley has become the ninth player to leave Forest this summer.
The midfielder, who has seen 15 signings come through the door at the City Ground, has joined Sydney FC after four-and-a-half years with the club.