General manager Steve Kerr says an MRI exam on Saturday showed a bulging disk, and the 7-footer will see a back specialist Monday.
Kerr says Lopez has had back spasms the last few days, leading to the MRI. Lopez will not rejoin the team at least until tests results return.
As Kerr put it, "We don't want him on an airplane right now."
The Suns, winners of six in a row, play at Minnesota on Sunday.
The insertion of Lopez into the starting lineup is a big reason the team has played so well lately. The Suns have won 20 of their last 25.