Mourinho was desperate for an experienced centre-half in what would end up being his final summer window at the club.
Boateng was among the names discussed near the end of that transfer window, but United blocked any deal.
The Athletic reveals that United looked at software that assessed Boateng's game and showed a decline in performances, along with significant injury problems.
It meant that a possible £13.1 million transfer was axed.
The data showed that Boateng was not fit enough to perform at his best level for the full 90 minutes, as he was making fewer tackles and running significantly less in the latter parts of games.