Castillo has been dealing with a sprained MCL in his right knee since Friday but hasn't made enough progress with the injury.
The only two catchers on the active roster now are Blake Lalli and Koyie Hill, who was obtained from the Cincinnati Reds on Saturday for cash considerations. Lalli was set to make his first career major league start Tuesday night.
What made the Cubs comfortable in putting Castillo on the DL was the fact that Steve Clevenger has been making progress in his rehab from a side injury in April. Clevenger was the designated hitter for Triple-A Iowa on Tuesday, going 1-for-2 and scoring a run from first base.
"If anything unfortunate happened to our other two (active) catchers we can just bring him here and he only has to be used in an emergency situation," manager Dale Sveum said. "We felt Welington wasn't really going to be ready to play and test that thing for 120 pitches for another four days anyway. It just seemed the logical thing to do."
Wood is taking over the rotation spot left vacant when Chris Volstad was optioned to Triple-A Iowa last week. Volstad opened the season 0-6 and the Cubs lost all eight games he started. He picked up a victory at Iowa on Tuesday.
The left-handed Wood was 3-3 with a 4.57 ERA in seven starts at Iowa and in his one major league start this season he allowed three runs over six innings in a May 6 no-decision to the Dodgers, a game the Cubs eventually won.
In his only career start at Houston's Minute Maid Park, Wood pitched 6 2/3 scoreless innings on May 9 of last season while he was with the Reds and picked up the victory.