Rashford broke into the Manchester United first-team last season, scoring eight goals from 18 games.
But the forward has found life trickier under Jose Mourinho this campaign.
Rashford has scored seven goals across all competitions and been ousted from his preferred position of centre-forward by Zlatan Ibrahimovic.
Mourinho has used the 19-year-old on the wing more than up top.
Manchester United's injury crisis
Mon, March 27, 2017
Manchester United may have injury and selection crisis when the Premier League returns this weekend
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Mourinho faces a possible injury crisis with a number of players unavailable
Marcus Rashford has endured a frustrating season at Manchester United
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Jose Mourinho has used Marcus Rashford on the left wing this season
And Mills believes he is not a starter because his pace frightens tired defenders late on in games.
Mills told the BBC: "A lot of speculation and talk will be about why doesn't he start more often for Manchester United.
"Why didn't Gareth Southgate start him for England?
"Sometimes players, when they are young, are better as impact players.
Marcus Rashford played as England beat Lithuania 2-0 on Sunday
"They come on and when the teams get a bit tired, there's a bit more space, there's a bit more freedom and they can run at people.
"He did it in the Iceland game at the Euros where he got three-and-a-half minutes to show what he could do.
"Gareth brought him on after 58 minutes against Lithuania and he looked lively and created things.
"That's what we want to see – and he's still only 19 years old."