The Toffees, who climb to fourth place in the table, left London with a point after substitute Steve Sidwell fired a 90th-minute equaliser that secured a 2-2 draw.
"I can't believe we only got a draw out of that, it's incredible," the Everton boss said.
"We've played really well this season but that one we should have won. We were terrific at times but couldn't get the goals.
"I'm really frustrated because if I really want to hang on to the boys at the top I need to win these games when they come along.
"To win away in the Premier League is really hard for any side and we should have won that comfortably but we didn't.
"Today you'd have to go away saying Everton look as smart as any side. That's the way we've been playing."