Having won the French Open earlier this month, Henman is pondering as to whether playing on grass is causing problems to his knee.
"It's a huge surprise but, for me, as big a surprise was to see him struggling with his knee because he's played nine tournaments this year - winning seven - and he's just come off winning his eighth Roland Garros," Henman told BBC Sport. "You can't do that if you're not fit.
"To see him struggle makes me wonder whether grass actually puts a bit more stress on his patella tendon, with maybe a bit more bending. Only Rafa knows if grass is a problem, but most of us would have thought that grass, being softer, was perhaps a little bit more helpful. Maybe that isn't the case.
"I tend not to want to speculate on these things. Rafa's pretty up front and honest, and I'm sure in the weeks and months ahead we'll find out a bit more. It shows the strength in depth of men's tennis at the moment and we should focus on Darcis's performance, it was phenomenal. There were definitely a few question marks about Rafa being seeded five, but not many people would have said this was a particularly tricky draw and he lost."
Nadal is currently ranked fifth in the world.