https://www.mlssoccer.com/post/2019/07/09/portland-timbers-gm-gavin-wilkinson-explains-summer-transfer-plans

"We will look to bring in at least one player," GM Gavin Wilkinson told MLSsoccer.com last week, adding that whoever is acquired will compete for a starting place rather than make up the numbers. "Obviously, at this stage of the season, it needs to be a player who can impact the results and the performance of the group. But we need to manage the performance of the group—too many changes is not healthy."

I think this pretty much has to be a defensive player. For all the Timbers' recent defensive solidity, I think you could definitely improve the backline with the right guy.