Zimmerman landed on the disabled list toward the beginning of the season with an inflamed joint in his right shoulder, forcing him to miss 13 games in late April and into early May, and has struggled to find his stroke since returning.
"I don't know if it's his approach or what he's thinking, but I know he can be streaky at times," Johnson said. "I sure need him to be comfortable, because he's invaluable to this ballclub. He can help out a whole lot.
"He's hard for me to read a little bit. He's really not attacking the ball. ... I don't know if his shoulder is bothering him or not, but we certainly need him."
Entering play Wednesday, the All-Star third baseman was batting .239, with three home runs, 22 RBIs and a .655 OPS over 46 games.
Zimmerman is coming off an injury-plagued 2011 that saw him post career lows in a number of offensive categories, but Johnson is fully confident the 27-year-old's production will get back to his pre-2011 numbers, when he posted consecutive seasons in which he hit at least 25 home runs and logged an OPS above .885 from 2009-10.
"He's too good a hitter to keep struggling," Johnson said. "We are at the early stages of the season, so there is plenty of time to pick up the slack."