In front of almost 1,800 fans on the South Coast, the Reds were the better side in the first half but went behind to a Lee spot-kick. Federico Macheda levelled matters on 81 minutes but all looked lost when Jesse Lingard was sent off for handling on the line soon afterwards. It was left to Johnstone, something of an expert at saving penalties, to emerge as the hero with a superb save.
The hosts almost went ahead in the first minute when a Ryan Tunnicliffe slip let in Emmanuel Mayuka but Johnstone made a decent stop to thwart the Zambian. Gradually, United gained control but passed up a host of opportunities to lead before the break.
Macheda probably had the best opening but fired the wrong side of the far post after a tremendous move involving Larnell Cole and Davide Petrucci. A Sean McGinty cross from the left brushed a post and Cody Cropper made a couple of good stops to frustrate the hard-working Angelo Henriquez.
Lingard, back from a loan spell at Leicester, also had a couple of chances but side-footed wide after good work by Henriquez and again worked Cropper while the keeper was extended when a Cole cross was deflected goalwards.
Despite all the pressure, the Saints improved in the second half and enjoyed a dominant spell. Ben Reeves' free-kick flew wide with Johnstone motionless and the home skipper then saw a shot deflect off target.
The penalty was awarded in the 55th minute when Macheda survived an appeal for handball but Lee