Johnstone is yet to make his first-team debut for the Red Devils, but has been one of the more impressive goalkeepers in the Championship this season, bouncing back from a difficult first loan spell to help the Villans put themselves in the play-off places after 17 games.
However, Steve Bruce may not have the 24-year-old available to him for much longer, with The Sun reporting that United have a January recall clause in the loan deal and they may use that clause with the Baggies considering a bid during the winter window.
According to The Sun, West Brom are keen on giving first choice Ben Foster more competition and may make a £5 million move for the stopper - though The Sun suggests that it is Tony Pulis who is a fan of Johnstone, so perhaps his own future will have a major say on how likely a move actually is.
A permanent switch to West Brom would leave Villa with a problem to address regarding their goalkeeper situation. The Sun reports that the Villans are keen on Newcastle United stopper Karl Darlow, who could be allowed to leave St James' Park for the right price.
It will be interesting to see how Johnstone feels about a potential switch to The Hawthorns as it appears that he would be second choice behind Foster at the start of the spell, and given that he has been number one for Villa, that switch may not particularly tempting for him.
Given how well Johnstone has performed at Villa Park, Bruce will surely not be keen for United to bring him back in January if that is what they decide to do.
But if they manage to land Darlow, that would arguably still be something of a coup for the Villans as he was a key player in the Magpies' promotion push last season.