Personally, I'd like to see Leanord Nimoy or Sean Connery briefly come out of retirement to voice Sentinel. Ian McKellan, Christopher Lee, or Patrick Stewart wouldn't be bad either.

 

I'm sure he'd be able to give the character a different voice than Optimus. It also saves some budget because that way they don't need to hire another voice actor...and other voice actors from the previous two movies also did multiple characters.

So in short: yes. And he really does look like Cullen..so if it was intentional, I don't see why he shouldn't voice the character.

Or they could let Frank Welker voice him