United were hammered 4-1 by Newcastle on Sunday.
Keane said on Sky Sports: "Not good enough, the second half...that was 14 league games they've lost now and we keep making excuses for this team.
"There's not enough players who can run in the team and I don't think there are players who want to run.
"Not enough of a goal threat...you've got to dig deep even when you're up against it and they didn't do that in the second half. We spoke at half-time, the big test was to come in the second half.
"Looking at Newcastle, Newcastle were outstanding, they are used to winning big games now they have got in a really good habit. United are the opposite. I think United now are physically and mentally a weak team.
"Newcastle were too fast, too powerful and more determined. The pace up front, United couldn't deal with their attacking players and they got what they deserved.
"Well done Newcastle they were outstanding but United - there was talk before the game about 'there's good signs for this United team and signs they were getting better'.
"Well, I can't see it, and I saw there another poor performance particularly in the second half. Not good enough and I worry for this group of players, for me they don't look like a strong group mentally."