21st May 2018, 11:28 | Leeds
Keinhorst, 27, who is capable of playing at centre or in the back row, made three appearances for the Vikings, scoring his first try in Saturday's 38-18 loss to St Helens.
Widnes head coach Denis Betts said: "We would like to thank Jimmy for his time and efforts at Widnes Vikings, and wish him all the best for the season ahead.
"We are also grateful to Leeds Rhinos for facilitating this loan."
Leeds lost captain Kallum Watkins, full-back Jack Walker (both knee), hooker Matt Parcell (shoulder) and prop Nathaniel Peteru (biceps) to injury at St James' Park, where they lost 38-10 against rivals Castleford Tigers.