The off-spinning all-rounder will face the new ball alongside captain Stephen Peters for the Division Two clash in Chelmsford that starts a busy programme of five games in six weeks.
As things stand, the County are second in the table, 27 points clear of their nearest challengers and 29 above Essex, who have a game in hand, in fourth.
And head coach David Ripley is keen to get stuck into the bread and butter of the first class game again.
He said: “Hopefully we’ll have a good, hard-fought game of four day cricket.
“Essex are right up there and will fancy their chances and Gautham Gambhir (Essex’s new overseas player) will give them soemthing extra.
“We’re looking forward to it after the success at the weekend.”
Middlebrook will replace Kyle Coetzer at the top of the order after the Scotsman’s injured wrist failed to recover, and also missing from the game will be David Willey, who is on England Lions duty, and Cameron White, who has flown back to Australia.
Replacing them will be Rob Keogh and Ben Duckett, with the former batting at seven and the latter at five, but Ripley said that James Kettleborough had come under serious consideration.
“James has been getting hundreds in the Second XI which is what I want to see, and he was close to getting in,” Ripley commented.
“But Rob (Keogh) can bowl a bit iof spin if necessary and this will hopefully give him the chance to bed in and show what he can do.”
As for the decision to promote Middlebrook, Ripley added: “James did a good job when he opened against Lancashire and at this stage of the season, with Kyle out, it’s a good move for us.”
There’s more good news for the County with Australian all-rounder Trent Copeland, who got back into the country late last week, available for the next three Championship outings.
Northants: Peters, Middlebrook, Sales, Wakely, Duckett, Hall, Keogh, Murphy, Crook, Copeland, Azharullah.