The Scottish giants lost 3-0 at Valencia in the Champions League.
Naismith hit the woodwork twice during the clash at the Mestalla.
Naismith told Sky Sports: “I think it’s the worst we’ve played since the start of the campaign, we didn’t defend great.
“We started off pretty well and I had a shot – when I looked up it was heading for what I thought was a goal, but it hit the post.
“But the biggest disappointment is we lost a goal from a set-play, throughout the campaign we’ve been good like that.”
