Said it in the match thread but will say it here too. Henderson isn't under much scrutiny there, he's young, first year in the prem and he's impressed. But being goalkeeper for Man United is a different universe to that. Every action will be scrutinized, everything will be deemed as saveable, and he has to stop them without making mistakes. Consistently. Thats what de gea did for 4 years. He stopped what looked impossible, consistently. He made crazy saves routinely and he had a long stretch without making mistakes. Now he's had a bad year and a half, but it doesn't mean that Henderson is ready to come in right away.
Henderson has had mistakes as well and even others he's let in would be deemed as mistakes were he at United. More hype, more attention and more criticism inevitably. Another year at Sheffield Utd will do him a world of good as he'll need to build on this season and cut out the mistakes that he's had before coming to United. Show he can handle the criticism and higher pressure. He'll be taking over from one of our best goalkeepers ever, and people will be remembering performances like De Gea vs Spurs, Madrid or Arsenal every time Henderson doesn't do the same. So yeah, its too soon for Henderson to just step in. Goalkeeper isn't like other positions, but thankfully I don't think there's a chance that Ole and the club rush into a decision like that.