With the Reds needing a fresh foothold back into the game he made the call and introduced Marcus Rashford with 20 minutes to play.
There were calls for Mourinho to take off a subdued and out of sorts Wayne Rooney but just when United looked set to drop their first two points of the season, it was the Old Trafford captain who created the winner.
Mourinho can do no wrong so far and this was certainly the biggest test of his tenure to date.
The Reds had not exactly had their own way completely in the previous matches against Bournemouth and Southampton with both the south coast sides starting well early on.
Mike Phelan's Hull asked far more questions of Mourinho's new-look team.
They were snappy in the tackle, looked a threat from set-pieces and on occasions even managed to play around Paul Pogba in midfield.
In the last few seasons, opponents would have made hay as chins hit chests.
But this isn't a United side who'll sag any more when the going gets tough- with characters like Pogba and Ibrahimovic that's unlikely to happen.
Whereas last term you ripped your hair out wondering if an opposition keeper might actually get tested, Hull's Eldin Jakupovic couldn't enjoy the same comfort.
You had to wonder if the adrenaline from a visit of United, plus a 100% Premier League start for Hull would wear off at some point.
But they didn't flag and what they lacked in numbers they more than made up for in spirit and dogged determination.
Henrikh Mkhitaryan's darting and penetrative runs began to cause Hull problems and forced both David Meyler and Tom Huddlestone into fouls on the silky winger in desperate last-ditch attempts to stop him.
United wouldn't give up and you have to be grateful for one of the old traits that Mourinho looks like he might have revived.
The Reds kept pressing as Hull had to descend into lashing balls into no-man's land to relieve the pressure with United hoping they might switch off for one critical moment.
When four minutes was held up on the fourth official's board, a groan went up from the home supporters who knew this was a different United side from the one that toiled so much last term.
Everyone knew what was coming and it did.