The Croatia international was wanted by Chelsea before the start of the season, but Spurs refused to do business with their London rivals.
The 26-year-old was understood to have submitted a transfer request and Pienaar knows it is difficult for a club to retain a player when he wants to move.
The South African ace is now back with Everton following a less than successful year-long spell at White Hart Lane and can see the argument from both sides.
Pienaar can see Spurs' need to keep their best players in the squad, while he also believes players can sometimes only have a few opportunities to move on during their careers.
"Luka is a top player for Spurs and if he leaves it is hard to say what will happen there," Pienaar told the Sunday Mirror.
"He is such an important member of the team. Tottenham will want to keep him at the club, but it is difficult when a player like him wants to move on.
"As a player only a few opportunities to move on come up in your life, so I can see it from both sides.
"It was hard for Luka at the start of the season because he was thinking about his future. But he is a good professional and after the chance to leave Spurs passed he did everything he could for the team. He is that kind of guy.
"That's why the other players like him so much. But I understand from my own experience that if a good opportunity is blocked by your club, it can be hard to concentrate."