And judging by the performance against Brighton at the AMEX on Sunday afternoon, it looks like he is right on the money. Neither did Paul Pogba's late penalty after Duffy had brought down substitute Marouane Fellaini in stoppage time.
It's not quite that bad for Mourinho but his preseason prediction is already coming true.
Martial, who was heavily linked with a move away from Old Trafford this summer, was making his first appearance since his manager confirmed he had been fined two weeks' wages.
Slack defending from the likes of Victor Lindelof and Eric Bailly helped the The Seagulls pick up all three points at The Amex.
Instead, Brighton went in front when veteran striker Murray caught Lindelof ball watching to flick home Solly March's cross with a classy finish.
Pascal Gross scores from the penalty sport to make it 3-1 Brighton.
Vladimir Putin dances with Austria's pro-Moscow foreign minister at her wedding
The pipeline will allow gas to be transported directly to Germany from Russian Federation without transiting through Ukraine. Merkel said a four-way meeting on Syria involving Germany, Russia, Turkey, and France is possible.
Sanchez, 29, did not travel with the team on Saturday afternoon after picking up a problem in training and is now a doubt for Tottenham's visit to Old Trafford on August 27.
Valencia suffered a calf injury during preseason while Matic underwent surgery on an abdominal injury picked up during the World Cup.
Chelsea will also be looking for a stronger finish than last season under new manager Maurizio Sarri, while Unai Emery plans his own title charge with Arsenal in north London after taking over from Arsene Wenger.
Luke Shaw and Ashley Young were handed starts in the full-back areas, and between them they lost possession 37 times (Shaw 15, Young 22), but that statistic can be a little misleading as they often tried to make things happen down the flanks, but failed to help break down Brighton's defence. It was a 1-0 win over United here in May that secured their top-flight status last season and this could turn out to be just as significant.
The Red Devils have always maintained that they have no intention of parting with a man who has been a model of consistency for them during a testing period - with that form recognised with a record-breaking haul of four Player of the Year awards. Are they good enough to stay in the top flight?