TCAN4551-Q1: Minimum Vio to use external crystal

Tool/software:

Hello - we are evaluating the TCAN-4551-Q1 for use alongside an MCU whose maximum GPIO voltage is 2.6V.  Initially we planned to use an IO voltage of 1.8V and an external crystal but read that due to the power requirements for the crystal driving circuit in the TCAN, it is not possible and an external clock source is needed.  

We are unsure whether the MCU can provide the necessary 20Mhz clock signal at this time, and wondering, would a IO voltage of 2.6V permit the use of an external crystal? Or is 3.3V the minimum voltage that has been qualified.  

thank you