Manchester United boss Jose Mourinho has criticised Henrikh Mkhitaryan's recent form - as Borussia Dortmund indicate they would be interested in re-signing the Armenia forward.
Mkhitaryan, who joined from Dortmund in July 2016 for £26.3m, has been left out of United's last two match-day squads.
The 28-year-old will return to the United squad for Saturday's Premier League game against Brighton.
Mourinho said Mkhitaryan's performances have been "decreasing step by step".
Mkhitaryan started 10 of United's first 11 Premier League games before he was dropped after being substituted in the 62nd minute of the 1-0 defeat to Chelsea on 5 November.
"I was not happy with his last performances," said Mourinho of a player who has scored twice in 16 games this season.
"It's not one or two, I am talking about three, four or five."
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Mkhitaryan made a modest start to his United career and did not start a Premier League game in almost three months after being substituted at half-time in the defeat to Manchester City last September.
His form improved during the second half of the season and he scored as United beat Ajax in the Europa League final in Stockholm in May.
"He started well this season but his performance levels, goalscoring, assists, high pressing, bringing the team with him as number 10, were decreasing step by step," added Mourinho.
"That was enough [to be dropped] because the others worked to have a chance. Everybody works to have a chance. It's as simple as that."
Dortmund have a history of buying back big-name players they have recently sold.
Midfielder Shinji Kagawa returned to the German club in 2014 after two years at Manchester United.
Germany playmaker Mario Gotze arrived back in Dortmund in 2016 after three seasons at Bayern Munich while Turkey midfielder Nuri Sahin had a season at Real Madrid and a loan spell with Liverpool before returning, initially on loan, after a two-year absence in 2014