Treble-winner Cole speaks from experience, having had to accept spells out of the side himself during his Reds career, due to the wealth of forward options at the club.
When asked about Kagawa at a club event in Tokyo, Cole said: "He is a quality player but he is not the only one who is finding it hard to get regular football. There are many top players in the squad and there is plenty of competition.
"But having seen Shinji play at Dortmund, I know for sure how good he is. He was the best player in Germany for two years. I believe he’ll come back with some good displays."
Cole says United should still target a Champions League spot with 11 league games to go.
"We need to win as many games as possible and a top-four finish is still a realistic target," he told ManUtd.jp.
"For whatever reason, it has been a tough season. But all the top teams will try to get better and we need to stay with them next year."