A poor first 45 minutes saw the Whites trailing by three at half time, but they regrouped in the second half and clawed the deficit back with a goal from Alexander Kacaniklic.
"We gave them too much space and they cut through the defence like a knife through butter," the 57-year-old told Sky Sports News.
"We were better after that, and we showed a lot of pride and character, and we tried to show people we can play. We showed some good performances in the second half, but not enough to win against Man Utd.
"If we had scored a second goal, we could have maybe got a result. The second half was probably the positive thing today."
The Dutch coach is under pressure at Craven Cottage as his team sit 15th in the Premier League.