Weston boss Frank Gregan was a busy man last Friday, sealing deals for both Barnett and McGregor.
Barnett has signed on work experience terms from Rangers. McGregor has joined on loan with a view to a permanent move from Conference side Tamworth.
But while McGregor will play at Sixfields on Saturday, Barnett will not be involved.
"I watched him play for QPR Under-17s on Saturday and he's one for the future," said Gregan. "He's a talented lad but I just think it's a bit early for him to play at our level. I won't be using him against Northampton."
McGregor had a loan spell at Woodspring Park earlier this season and Gregan is delighted to have secured his services permanently.
"There is still work to be done on the deal so we've signed him on loan for the FA Cup game," added Gregan. "When we tie up the loose ends Mark will be our player and he'll be a real asset.
"I think Mark McGregor is coming back to his best form."
The recruitment of McGregor should allow Gregan to switch Matthew Rose back into midfield on Saturday. Rose scored the winner against Bognor Regis in the FA Trophy playing as a striker.
"Matthew has done really well up front and I'm not surprised because he started out as a striker," said Gregan.
"He moved into midfield because he's a big, strong, lad and has made that his niche."
McKeever has not played since Weston's first-round win at Farnborough because of a hamstring strain but is now ready to return.
"Everyone knows how important Mark is to us and with another week's recovery he should be ready to go," said Gregan.
Northampton lost 3-0 at home to Mansfield on Saturday, but Gregan knows his team face a tough test.
"I was very impressed by the set-up there," he said. "I've seen worse grounds in the first division. And Northampton will be tough to beat.
"They are a well-organised, strong side and on Saturday just came up against an awesome performance from Mansfield."
Gregan's options are further reduced as both striker Paul Hunt and midfielder Mark Cherry are cup tied. But Steve Benton is back from suspension to play in defence.