Conor Gallagher scored the game's only goal when he drove through a crowded box early in the first half.
Bilic said of his goalkeeper Johnstone: "We're a team that's going to need a goalkeeper to be there in every game. Sometimes it's going to be one save like against Burnley, sometimes it's going to be four like against Man United, sometimes it's going to be two or three like tonight. Sam at the moment is doing brilliantly for us and we need it. We don't have the quality that's going to keep them without having one or two chances.
"Against Spurs we kept a clean sheet until a few minutes to go and against United we conceded from a penalty, so it's a great basis for us."