Bulls big man Bobby Portis will get his opportunity to start Monday against the Celtics. But Denzel Valentine will finally get his chance to shine. With all the talk out of practice the last few days about Portis taking his instant energy from the bench to the starting lineup, one very important question had to be answered Sunday: Who picks up the slack? Portis was averaging almost 17 points per game off the bench since the start of February, and it isn’t like the coaching staff is counting on Cristiano Felicio to pick up that scoring with the second unit. That’s where Valentine comes in. Like Portis, Valentine is a first-round draft pick (14th overall in 2016) looking to make a lasting impression and be part of the rebuild. The small forward/guard started 32 games in the first half of the season while Zach LaVine was working his way back from knee surgery, but now he’ll help carry the second unit. “Definitely, I’m going to have to be more aggressive,’’ Valentine said. “Different rotations, guys being hurt and in and out of the lineup, and then the whole lineup change after the All-Star break, it’s been really difficult personally for me dealing with all this. But I’ve been given a great opportunity to showcase what I can do and try to help the team. “It always works itself out. It’s going to be just fine. We’re going to play with energy and as hard as we can. It’s a great opportunity for us to play hard and finish out the year the right way.’’ That sounds good, but when coach Fred Hoiberg was asked if he was concerned about the second unit — especially offensively — with Portis no longer part of it, there was little hesitation in his reply.