The Cuba international, 28, has agreed a season-long stay at the Championship club and was in the stands for Boro's 1-1 draw with Blackburn on Saturday.
Hernandez has twice helped the Canaries win promotion from the Championship and played 26 times in the Premier League for City in the 2019-20 season.
He joined Norwich from Braunschweig in January 2018 and was the first Cuban to play in the Premier League.
He started just six times in the Championship as Norwich made an immediate return to the top flight last season, featuring in 15 more league games as a substitute.
"I think he needs us as much as we need him, having not played an awful lot over the past year.
"He'll get plenty of opportunities here and I'm looking forward to working with him."
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