Midfield: Paul Pogba inspired United's stunning comeback against City last week and the French worldwide is expected to keep his place in midfield following his two-goal display.
Rashford has started just 30 of the 70 Premier League games under Mourinho, while Martial appears to have dropped down the pecking order since the arrival of Alex Sanchez in the January transfer window.
"It is just football, we can not have 11 players, it would be wonderful economically for the football clubs to have only 11 and play with the same 11, no injuries, suspension, fatigue, it would be better for everyone to have only 11 but is not possible".
"If I was you I would go in another direction", Mourinho told reporters ahead of Sunday's game against bottom side West Brom.
Goalkeeper: David De Gea produced another excellent performance during the win at Man City last weekend and he's certain to keep his place between the sticks with Sergio Romero out with a knee injury.
"It is just football, we can not have just 11 players". I've had one before (a long injury), but now it's OK.More news: Sloppy start against City frustrates Spurs boss Pochettino
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"So at every club at every level, from the top of the world until the amateur football clubs you need more than 11 players and only 11 can start, it is as simple as that, there is nothing I can do".
However, the Special One didn't get a chance to see the youngster in action then, as the 20-year-old withdrew from the matchday squad.
City needed a win to secure a third Premier League title in seven years but despite roaring into a 2-0 half-time lead, United hit back to claim a 3-2 win and put their derby rivals' celebrations on ice.
"To finish fourth we need four or five points, but to finish second we need much more than that".
Speaking to Spanish newspaper Mundo Deportivo, Herrera said: "Mourinho backs you up if you give it your all and that's how I understand football".