Brandon Allen was Burrow's backup for the past three years, but he split for SF. Bengals just signed Trevor Siemian to compete with the pride of the UW Huskies Jake Browning for the backup QB gig.
Browning has been on the Bengal taxi/practice squad for a couple of years, but he hasn't appeared in a NFL game yet.
Siemian did a pretty decent job in Denver for two years (2016-2017) considering he had next to nothing to work with.
A Pop Warner backfield and one dependable receiver (Emanuel Sanders) and managed to go 13-11 with garbage.
Problem is Siemian is an injury waiting to happen, and you have no idea what Browning can do if he ever does get a start.
(the feeling here is not much, like I mentioned, he's never taken a snap under center, and he'd be playing against some mighty stout defenses)
It's all a moot point anyway, Burrow's target date to return is 8/12, Siemian will more than likely take Brandon Allen's back up role this season, and Browning back to the taxi squad.