Shiels was let go by the Ibrox outfit following promotion.
Shiels spent four years at Ibrox
Shiels, who signed from Kilmarnock four years ago, is one of four senior players to be culled by Mark Warburton - despite being a regular in last season's Championship-winning side.
The Northern Irishman appeared 31 times en route to the tile, scoring twice and providing one assist, though his father, Kenny Shiels, believes he knows the reason for his son's departure.
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Shiels Snr's position as manager of Derry City has prompted talk of a family reunion at the League of Ireland side this summer, however, he stressed his son will not return home at this moment in time.
"He’s got opportunities to go to the continent so I’m sure he’ll take one of those up," the 60-year-old added.
"It’s not a runner with us. It’s not feasible or possible at this moment in time."
Shiels Jnr, who also played for Hibernian and English side Doncaster Rovers, was part of three promotion-winning sides at Ibrox.
In total, the 31-year-old played 125 times for Rangers, scoring 32 goals and providing 23 assists.
He leaves alongside long-term teammates Nicky Law, Nicky Clark and David Templeton at the end of their contracts.
Law is another to be shown the door by Warburton