Villarreal announced the arrival of Alcacer on Thursday, with the former Barcelona striker signing a five-and-a-half-year deal and reportedly commanding a club-record fee of €23million (£19/$25m).
Eleven of those goals were scored after coming off the bench, setting a new Bundesliga record for goals by a substitute in the process.
That flying start prompted Dortmund to turn Alcacer's loan into a permanent five-year deal in February 2019
However, Alcacer has struggled to replicate that form this campaign. He has made just nine starts in all competitions under Lucien Favre this season and struggled with injuries, while the January arrival of Erling Haaland from Salzburg led to increased competition for minutes.
Alcacer - linked with former club Valencia earlier in the window - has consequently returned to Spain with Villarreal, who are eighth in La Liga and booked a place in the Copa del Rey quarter-finals on Wednesday.
It may not be the last of Dortmund's activity in the transfer window, with Can close to arriving from Juve.
The 26-year-old has fallen down the squad pecking order since Maurizio Sarri's arrival as Juve coach in the summer.
As a result the Italian champions are willing to sanction the former Liverpool midfielder’s departure in the winter window, but only if their reported €25 million (£21m/$28m) valuation is met.
Dortmund sporting director Michael Zorc told a news conference on Thursday: "Well, the transfer window isn't open much longer and I don't like to provide any updates until something is final.
"We are talking with the player, this is no secret. But a decision hasn't been made yet."