Giggs was forced off shortly before half-time against West Bromwich Albion, and United missed him badly in the second half.
The 36-year-old had trained all week and looked to be over the hamstring problem, but he suffered a relapse and he will need to be rested.
“The loss of Ryan as a wide player was a big one for us,” Ferguson told manutd.com. “He’s done his hamstring again. He got it against Bolton but he’d trained all week. He’d been doing very well, but it’s gone again and maybe we need to give him a longer recovery.”
Giggs’ departure from the game made a real difference as United didn’t have a natural winger on the bench to bring on in his place.
“We had a problem with Ryan coming off,” explained the boss. “We maybe didn’t have enough cover in the wide positions. Bebe had picked up a knock. He’s OK, but we decided to leave him out.”