Newcastle were hammered 4-1 on Saturday, as Bruno Guimaraes' goal was swamped by a Justin Kluivert's hat-trick and Milos Kerkez's late strike.
Howe said: "Firstly, I have to congratulate our opponent. I thought Bournemouth were better than us today.
"I thought they played very well and never allowed us to find our rhythm in the game. I am disappointed with our performance, that is not the level that we played at in the last group of games.
"I thought we were devoid of energy today. We had a real problem athletically. We seemed fatigued and we never really recovered from a poor start to the game and ultimately have a let to reflect on."
Howe added: "My initial assessment is fatigue. That is not making excuses. Bournemouth were better than us, but from our point of view, the three games in six days meant we were lacking in that sharpness and sprinting that they had. I may feel differently when I watch the game back. But that's how I feel now.
"It was difficult to change the team after winning nine in a row. I don't want to beat myself up about that it is a difficult one. It was difficult for me personally to think about rotating the team in this moment. But even then with our substitutions they did not make an impact either."
Howe also said: "Yes we lost three points but maybe it is an important marker for us to know that if you drop you level against anybody at this level you will get punished. That is the first time we have done that for a long time home or away.
"Our qualities have been on show (lately) but I don't think any of our game survived - we were off in every department. When you perform like that you aren't going to get anything from the game."