Manchester United had identified Chelsea defender Emerson as a potential alternative to Alex Telles during the closing stages of the transfer window.
The Red Devils were in regular contact with Porto regarding Telles, but it initially appeared that they could miss out on the left-back.
However, they were eventually able to find an agreement for the Brazil international, who was signed on deadline day.
According to Fabrizio Romano, the Red Devils had earmarked Emerson as a potential alternative, had they failed to sign Telles.
Emerson was linked with a possible move away from Stamford Bridge this summer with plenty of interest from the Serie A.
Both Juventus and Inter were fancied to sign him, but neither were willing to negotiate a permanent deal for his services.
With the transfer window for international deals closed, he may have to wait until January for a move unless he is prepared to drop to the Championship.
The domestic-only window is open until October 16. Premier League sides can only trade with EFL teams during this period.