Johnson raised concerns that Houston could end up in violation of its deal with the NBA's Rockets that no other municipal venue would be used to compete for major events through 2013.
The Harris County-Houston Sports Authority on March 25 voted to proceed with the project. The plan still needs city council and Harris County Commission approval.
The Dynamo would pay $60 million of the cost of the stadium, planned for a vacant lot east of downtown Houston. The city and county would pay the rest, with a cost estimate of at least $20 million.