Manchester City star Ilkay Gundogan has told Sport Bild that Manchester United could "100 percent benefit" by playing Bastian Schweinsteiger and suggested the midfielder's treatment had been "disrespectful."
Schweinsteiger joined United last year during Louis van Gaal's reign but has been frozen out since Jose Mourinho's arrival in the summer and was written off as an asset in the club's accounts last month.
Gundogan, who played alongside Schweinsteiger for Germany before the 32-year-old's international retirement, said: "I believe that Manchester United could 100 percent benefit from a fit Bastian Schweinsteiger."
Schweinsteiger, who last played for United in March, has remained supportive of the club despite apparently being made to train with the reserves.
Gundogan added: "If everything you hear and read is true, you can consider it disrespectful. He's not just some player, and also not only 18 or 19."
The 25-year-old City midfielder, who moved to the Etihad Stadium from Borussia Dortmund in the summer, is a candidate to fill the void left by Schweinsteiger's international retirement.
"I trusted myself to do that before [Schweinsteiger's retirement], but this has nothing to do with saying his retirement is positive or negative," he said. "We should be thankful that we had such a great player."
Former Germany defender Per Mertesacker, meanwhile, said Schweinsteiger's decision to leave Bayern Munich had proved unfortunate given Van Gaal's departure.
"It's bad luck for him that he joined a club that has again changed its manager, and that the new manager does not want him," the Arsenal captain told kicker. "It can happen to you at any time in this business, and it's a bit perverse, but Basti signed a good contract, and he knew where he was going.
"He just had bad luck with the way it worked out. He will draw his conclusions and I wish him all the best."
Schweinsteiger said in August that United would be his last club in Europe and that he would be ready to represent the club if required.