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Mkhitaryan started the season in fine form, making five assists in the Red Devils’ opening three Premier League games.
But the Armenian has since fallen out of favour and has been overlooked by Jose Mourinho for seven of United's last eight matchday squads.
His most recent appearance was as a substitute in the 1-0 win over Brighton on November 25.
Mourinho also admitted other players in his squad are more deserving of a starting place than Mkhitaryan based on current form.
And Smith thinks the 28-year-old could be sold in January to pave the way for potential new arrivals.
Manchester United’s likely line-up to face West Brom away
Manchester United FC
“He is a sellable asset and a bargaining chip perhaps for somebody else that Mourinho might want to get in”
Alan Smith
"Who knows what will happen in January? There will be plenty of offers for someone like Mkhitaryan," Smith told Premier League Daily.
"Whether they will be the right place or not is another matter. He is a sellable asset and a bargaining chip perhaps for somebody else that Mourinho might want to get in.
"I still think he would be reluctant to sell somebody like that, who is a quality player when he is playing with confidence."
Despite his recent struggles at Old Trafford, Smith believes the former Borussia Dortmund star still has a future at the club.
"He has not been playing well enough but maybe the system doesn't suit [him]," Smith added.
"He [Mourinho] has been relying on the likes of (Anthony) Martial and (Marcus) Rashford in those wide positions. Jesse Lingard has also been showing good form.
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"Mkhitaryan hasn't been playing well enough. He is a very talented boy but is yet to show us on a consistent basis what he showed at Dortmund, for instance.
"I don't think Mourinho will be giving up on him just yet. It shows the depth of their squad really in terms of talent, that he hasn't made the matchday squad a few times."
Manchester United travel to face West Brom on Sunday.