The 20-time English champions kicked off their pre-season preparations with a 5-2 win over Los Angeles Galaxy on Saturday night, and Mourinho played three at the back in both halves of the contest.
A 3-5-2 formation was used by Mourinho in the Europa League last term, and the head coach's comments have suggested that a change in system could occur during stages of the new season.
"We have trained for five days and in these five days we trained for two days in the relation to this [three at the back] system," Mourinho told reporters.
"The improvements are in relation to four hours of work so I couldn't expect much better, it's something we have to keep working on in case during the season we decide to play three at the back. We did it twice against Rostov and maybe we want to do it more times."
United will continue their pre-season tour of the United States with a clash against MLS outfit Real Salt Lake on Monday night.