Might as well sell Dalot if we sign Wan-Bissaka — both of them are very good fullback talents and neither will be pleased with a stop-start role in a crucial phase in their development. Players from a weaker league usually take time to acclimatize to a higher standard, especially one who has played less than 20 first team matches at Porto and United combined like Dalot...either put your faith in him as the nominal starter (and potential right-back mainstay for the next decade) and surround him with an experienced head who can act as
bridge option while the former grows in stature (bearing in mind that Dalot was coveted by a host of clubs for a reason and was an elite player in the youth circuit), or get rid and sign Wan-Bissaka (or possibly Hakimi if Madrid are amenable). I'd probably sign Alves on a 1-2 year contract and go with the former option — think Dalot could learn a lot from a serial winner and one of the best fullbacks in modern football, and someone with Alves' nous/experience/gravitas would be hugely beneficial for the squad in general. Or maybe Juanfran, whose contract at Atlético also expires in the summer...
https://www.marca.com/en/football/spanish-football/2019/01/01/5c2949b4e5fdeab0778b45bb.html
Neither will hamper Dalot's development and would probably be ok with somewhat diminished roles over the next couple of seasons considering their age profile.