McKinney signed a minor-league contract with the Yankees on Friday, Jack Curry of YES Network reports.
McKinney (back) elected to become a free agent Monday.
The Yankees outrighted McKinney (back) to Triple-A Scranton/Wilkes-Barre on Thursday, Joel Sherman of the New York Post reports.
The Yankees placed McKinney on the 10-day injured list Tuesday with a left lower-back spasm.
McKinney (back) will start in left field and bat seventh in Sunday's game against the Red Sox.
McKinney's absence from the lineup Saturday against the Red Sox is due to a stiff back, Bryan Hoch of MLB.com reports.
McKinney went 1-for-4 with a solo home run in Thursday's 4-3 win over the Astros.
McKinney is out of the lineup for Monday's game against the Rays.
McKinney is out of the lineup for Wednesday's game against the Mets.
McKinney went 1-for-2 with two walks, two runs and a stolen base against Kansas City in Saturday's 5-2 win.