Pacquiao completed his second sparring session near Manila on Tuesday but his form five weeks from the bout has failed to impress.
"His second day of sparring today is maybe the worst I've ever seen so far," trainer Freddie Roach told Filipino media.
"His timing is really, really off."
The comments will be music to the ears of Horn, the 29-year-old former schoolteacher who is plotting what would be a monstrous upset over the eight-division world champion, who holds the WBO welterweight title.
Pacquiao, also a senator in The Philippines, sparred five rounds against two local fighters, who both reportedly held their own against the boxing great.
