The Saddlers have been left short in that department with Aaron McCarey being recalled by Wolves. Johnstone goes straight into the Sadler's squad that lines up against Hartlepool United on Saturday.
Johnstone is well thought of at United and tipped to have a bright future ahead of him after making a number of impressive performances for the Under 21 side. But Ben Amos was recently recalled from Hull so now Johnstone must go out on loan.
Oldham and Scunthorpe both had the benefit of his presence last season when he went out on loan to both clubs.
Walsall boss Dean Smith told the club's official website: "It was unfortunate that Aaron was recalled, but he will be replaced by a very able lad in Sam Johnstone.
"I know that Manchester United have high aspirations for him and he has already had a couple of previous loan spells at this level, so he knows what it's all about.
"I know a lot about the lad and watched him in action recently. I've had some very positive reports on him and am delighted to be able to bring him on board until the end of the season."
Source: United Mad