Meta there is still a difference between Iniesta as a sub and Bojan. Bojan is naturally a goalscorer, it's harder for a player like him to come on and create something out of nothing, which Iniesta has always had in his locker. I'd also say Frank's use of Iniesta was far better than Pep's has been of Bojan.
Even when Bojan has been fit, right across Pep's 3 seasons with the club, he's had 5-10 minute cameos at the death of games, when the team is at half pace and the game is well over. Iniesta had those appearances as well, but he also had a a lot of 20-25mins opportunities, and he came on in basically every game. I'm sure in one season under Frank he appeared in every league game?
Bojan needs regular first team football, the likes of Valencia or Villareal would be good moves for him. It's no coincidence that his best form since he made his debut was when we were forced to play him due to injuries etc. Frank gave him the chances and a run in the first team, and he was excellent. He needs that again, and he won't get it at Barca unfortunately.