The midfielder was expected to play a big role at Camp Nou this season after arriving last summer in a deal that saw Arthur move in the other direction to Juventus.
However, he has started just five LaLiga games for Barcelona, and has admitted he doesn't agree with Koeman's decision to not use him more.
What has Pjanic said
In an interview with Telefoot, the Bosnian said: "I have played at all the clubs I've been at and with all the coaches I've had. A situation like this has never happened to me. Of course it’s not easy, I accept it with great difficulty.
"I don't have the exact reasons why I've not had the playing time I expected.
"I keep working and you have to respect the decisions that are taken, even if you don't agree. I want to leave my mark at this club."
Pjanic's first season at Barca
The 30-year-old was a key figure at Juventus for four seasons, and with Roma, Lyon and Metz before that.
However, Pjanic has often found himself on the bench under Koeman. He has started all six of his team's Champions League matches this season, but just five of 23 league games.
Recently the emergence of youngsters Riqui Puig and Ilaix Moriba in midfield have dealt a further blow to Pjanic's chances of regular minutes.
Pjanic has yet to register a goal or an assist this season. Only once in his career – in 2008-09 with Lyon – has Pjanic gone through an entire season without scoring a goal.
What's next for Barcelona
Barcelona will face PSG on Tuesday in the first leg of their highly anticipated Champions League last-16 tie.
Pjanic will be hoping to continue his run of starting all of his side's Champions League matches this term, with PSG star Neymar set to miss out on the reunion against his old side after suffering an adductor injury.