Portuguese pair Dalcio and Fabio Cardoso were on the same flight from Portugal as Caixinha as they landed in Glasgow on Sunday night.
It is understood that 23-year-old Cardoso - who was first revealed to be a Rangers target by Sportsmail - would cost £1.5million, while Dalcio, 21, is expected to join on a season-long loan.
With Bruno Alves, the veteran Portugal defender, already having signed, Jack is the second of Pedro Caixinha's summer captures as the head coach's reconstruction of Rangers suddenly gathers pace.
A deal for Mexico global Carlos Pena is also in the pipeline, if a work permit can be obtained for the Chivas striker.
The latest report shows a photo of Herrera at Glasgow airport, landing after he attended the Champions League final on Saturday in Cardiff.
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The ECB left all of its main interest rates as well as its quantitative easing programme unchanged on Thursday as expected . While few expect that to happen, the words underline that the bank is not yet willing to call time on the stimulus program.
Talks remain ongoing to land Dalcio's Benfica team-mate Daniel Candeias for £700,000.
Veracruz forward Herrera played under Caixinha at Santos Laguna.
The central midfielder has signed a three-year deal at Ibrox after his Aberdeen contract expired.
Meanwhile, Rangers are understood to be closing in on the appointment of a new director of football.
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