DeGea has already extended his contract but I don't really know the plans of ETH, if United signs Andre Onana who is he going to use as his first choice goalkeeper. Onana is Inter Milan's first choice keeper, he's already doing well for himself on goal and will not like to come and play second fiddle to DeGea at United. I don't know if ETH is going to sign him and let him continue this season with Inter Milan because it will be disrespectful to bring him in and relegate DeGea to the bench.
I also didn't knew this but I am only posting this after checking- De Gea actually did agreed to extend but United backed out
[1] with even lower salary which is yet to be signed by De Gea. It does seems that United priority is Andre Onana right now as first team goalkeeper instead of De Gea. Well there are some harsh choices that a manager has to make and it has to be made in order to lift the level of performance of a club. It's a tough one but if this doesn't works out then he will also have to face criticism from United fans also.