I think it would be surprising if he ends up being a pure backup, like Telles. You could also imagine there might be a division of labor depending on the opposition or situation, with Trippier brought on or started when we need to emphasise offensive contribution.
A couple of thoughts on how various rumours here might be connected, assuming there's something to what we're hearing (which is of course a bit speculative). Apparently, we are in well-advanced talks to sign Sancho for around 80m. That's basically the whole transfer allocation. So, we're scrambling to find creatively cheap solutions elsewhere. Hence the possibility of moving AWB to CB and bringing in Trippier, which addresses needs at CB and RB for just 20m. Provided of course you think AWB would be an upgrade in central defence.