Grujic was looking for a permanent switch away from Anfield this summer, but a deal failed to materialise before the transfer window's closure as he faced the prospect of not featuring in top-flight football until January at the earliest.
While the English summer transfer window closed on Monday night, the Portuguese deadline runs to October 25 - allowing Porto to come in and swoop for the Serbian.
Grujic spent the last two seasons on loan at Hertha Berlin, where he impressed with his sharp thinking in the middle of the pitch. The Bundesliga side were hoping to strike another loan move this summer, but a deal couldn't be struck.
Similarly, a touted £18million move to fellow Bundesliga side Werder Bremen fell through on deadline day, with Porto pouncing the next day to secure the 24-year-old on loan.
Grujic has impressed during his two League Cup appearances so far this season, with some supporters hoping he would be given a regular chance to feature in the Reds' matchday squad.
But a departure was always likely, with Liverpool well-stocked with midfielders who are better-suited to Jurgen Klopp's style of play.
Additionally, the Reds can submit a maximum of 17 non-homegrown players in their Premier League squad - and Grujic was the obvious omission.
Should he have failed to secure a move away, it would have meant at least three months of playing football for Liverpool U23s, with the January transfer window his earliest opportunity to move.
But he will now have a chance to feature in the Champions League for the Primeira Liga champions.