That victory see Cardiff remain sixth, occupying the last play-off spot, five points ahead of United in seventh.
"It is a six-pointer. If we get the win then hopefully we can cement our place in the play-offs," said McCormack.
But McCormack, who played against Watford despite Michael Chopra being back from a two-match suspension, is well aware that Cardiff's form has been inconsistent this season.
"We go on little streaks, hot and cold, and that's not really good enough at this level," he admitted.
"You have to be consistent. You have got to grind results. But overall, I think we deserve a place but you don't get what you deserve in football.
"Hopefully we stay as a unit. Everyone is with each other - the subs, the injured players [and] the management team, and the fans were fantastic [against Watford], really roaring us on."
McCormack admits that so far it has been a season to forget for him personally, but he is hoping he is turning a corner.
"I've been wanting to get a run in the team and get in the goals and hopefully this will help me confidence-wise as well," he added.
"It's been a tough season for me with the front four playing really well and me being injured and off form. It's been a tough season.
"But hopefully now I can kick on."
And the Scottish striker says was glad that he was shown some faith after his performances against Leicester and Coventry.
"I think it would have been a little bit unfair if they left anyone out them two performances [against Leicester and Coventry]," he added.
"So I was just glad they showed me faith and that gave me a little bit of confidence.
"It was a hard game. I knew from the outset it was going to be tough. Watford are no mugs.
"But the boys played well and we fought and showed spirit. Second half we wanted to lift it. We came out of the traps, got the two goals.
"It was a good strike. I've got to give Whits [Peter Whittingham] a mention for the ball that he played [from the corner]. Pete's put it on a plate for me and I've just got to connect and luckily I did."
Source: BBC Sport
Source: BBC Sport