Herrera came on at half-time against Real Madrid in Santa Clara on Sunday but was forced after just six minutes on the pitch, leaving the stadium holding his waist following a heavy collision.
Mata has not played since picking up an ankle injury in the second half of the 2-1 win over Real Salt Lake.
Speaking about the duo, Mourinho confirmed they should return from injury in Sunday’s match against Valerenga in Oslo.
“Herrera and Mata cannot play tomorrow but they will recover in time to work next Saturday,” he said.
“The two injuries we’ve had were only small.”
After facing Barcelona in Washington, the side play Valerenga in Norway and Sampdoria in Dublin before facing Real Madrid again in the Super Cup in Macedonia on August 8.
It’s a massive blow to have Herrera and Mata both out, but it is only pre-season and what matters is that they’re fit for the new campaign.