Assaidi, who has won 22 caps for Morocco, had been linked with a move to Fulham before Liverpool stepped in to get a deal done. His signing-said to be worth £3 million, according to the Daily Mail-will follow those of Fabio Borini and Joe Allen.
Assaidi has scored 20 times in the Eredivisie since joining Heerenveen three years ago, including 10 goals last season.
The prospective signing of Assaidi could well mean the end of Liverpool's attempts to sign Barcelona winger Cristian Tello.
The Reds were said to have been in talks in recent days to take the 21-year-old on a season-long loan. However, Marca reports that Barcelona have offered Tello a new contract in order to ensure he remains at the Nou Camp.
Tello earned his breakthrough into the Barcelona first-team last season yet had only been offered a place in the B-team for the current campaign. The renegotiated contract is said to offer Tello a considerable pay rise which would appear to have assuaged the Spain Under-21 international.
While the prospects of one loan deal fades away, Liverpool could be about to complete another.
According to Marca, Liverpool has now emerged as firm front-runners in the battle to sign Real Madrid midfielder Nuri Shain on loan for next season. With Real likely to conclude a deal for Tottenham's Luka Modric in the coming days, Sahin's exit is thought to be imminent.
Arsenal have also been trying hard to bring Sahin in, but Brendan Rodgers' relationship with Real manager Jose Mourinho is set to prove decisive. Arsenal boss Mourinho did not have the best of rapports with Arsene Wenger during his time in England, while he has always spoken highly of Rodgers since the pair worked together at Chelsea.
Sahin was rated as one of Europe's top prospects when he left Borussia Dortmund after leading them to the Bundesliga title and being voted the league's player of the year. However, he struggled to earn a place in Mourinho's side last season and has been deployed at left back in preseason.
There have also been further reports on the future of Liverpool defender Daniel Agger. The Denmark international has been strongly linked with a move to Manchester City in recent times, with the Premiere League champions believed to have tabled a £20 million offer.
Despite earlier saying that every player had their price, Rodgers has been speaking of his eagerness to see Agger remain at Anfield.
"For me there has been no big change. The first point is we don't want to sell him," Rodgers said, according to the Independent.
"Secondly, I believe he will be a player who will thrive in this way of working and thirdly he doesn't want to leave - which is a great help as well.
"He is a top player and I want to keep him here. Over the last couple of months we have been able to sell the project and vision to a number of senior players.
"They are very happy to move forward with it and I would expect Daniel to be the same."