CHICAGO - Will Middlebrooks hit .297 with 16 RBIs in 18 games for the Red Sox when Kevin Youkilis was on the disabled list. The rookie third baseman also had 11 extra-base hits.

His playing time and production have dropped since Youkilis returned. But manager Bobby Valentine isn’t very concerned.

“His play has been OK,’’ said Valentine. “He’s faced some tough pitchers in and out of the lineup. Seems like he’s staying very competitive.’’

Middlebrooks has started 15 of 21 games since Youkilis returned. He is hitting .280 with six RBIs and two extra-base hits. Middlebrooks is 6 of 29 (.207) in nine games this month with one RBI.

“It’s a learning experience,’’ Middlebrooks said. “I want to be in the league, and this is the situation. Bobby is doing the best he can.’’

Middlebrooks has been an everyday player throughout his career, so coming off the bench or being skipped over on certain days takes some getting used to.

“You have to be strong-minded,’’ Valentine said “[Hitting coach Dave Magadan] is so good at the work he gives the guys. They do stay ready, as ready as you can.

“[Middlebrooks] is watching the game; he’s into the game. If I saw any elevated frustration level or any total decline in performance, I think we’d have to make another decision. But it doesn’t seem like we’re there.’’

Valentine talked to Middlebrooks when Youkilis came back, trying to lay out the parameters of the situation as best he could.

“I told him to fasten his seat belt,’’ the manager said. “That’s life in the big leagues, you’re in the fast lane, doing great.’’

Valentine said the 23-year-old Middlebrooks has matured enough as a player to stay in the majors, even if it costs him some playing time.

“If he was totally in the development stage of his career, it would be a slam dunk [to send him down],’’ said Valentine. “But because he’s over most of the development stage, he’s in the position that he’s in.’’