The 29-year-old, who has scored 19 goals for United this season, recently handed in a transfer request after talks over a new deal stalled.
A deal to take the Argentine to Carrow Road has been agreed between the clubs, with Canaries striker Steve Morison set to move to Elland Road in part-exchange.
Morison, who joined Norwich in the summer of 2011 from Millwall, was understood to have travelled to Yorkshire on Wednesday to undertake a medical.
The 29-year-old Wales international has struggled to break into the City first team this season.
Norwich have been on the look-out for a striker all month to help them in their bid to avoid a slump in form which has seen them drop to within seven points of the relegation zone.
The Norfolk club, who have had three bids for Celtic striker Gary Hooper
rejected, have only scored 25 league goals in 24 matches so far this term.
The club confirmed on Wednesday night that they had signed striker Kei Kamara on loan from Sporting Kansas City, subject to him receiving a work permit.
The Sierra Leone international tweeted on Thursday: "Am so excited for this opportunity to play for @NorwichCityFC. I cant wait to get out there, see everyone and feel the energy @ Carrow Road."