Very little that forward Lauri Markkanen has done in the last year has screamed ‘‘rookie.’’ From dominating for Finland in the European Championship last summer to stepping onto an NBA court and becoming a primary offensive option for the Bulls, it all has looked too easy. Until it doesn’t. After the Bulls’ 122-104 loss Saturday to the Timberwolves, Markkanen admitted for the first time that he was feeling the length of the NBA season and that his confidence had taken a hit. ‘‘It’s frustrating, not making shots, but it is what it is,’’ said Markkanen, who was 1-for-8 against the Timberwolves. ‘‘It’s a struggle, and I’ve got to work through it. ‘‘Obviously, it’s been a couple of games now. It’s been a long season, I’m not denying that. But I’m trying to work through it.’’ Markkanen, 20, not only has started 53 of the Bulls’ 59 games, but he became a father for the first time three weeks ago, then participated Feb. 16-17 in All-Star Weekend action. ‘‘He had a lot of really good games in the beginning, and it’s never going to be a 100 percent season,’’ teammate Zach LaVine said. ‘‘There’s so many games, you’re eventually going to run into a slump.