Philippe Coutinho "only counts as half a player" for Bayern Munich at the moment, according to Dietmar Hamann, who feels the attacker is not contributing enough going forward or in defence for his new club.
The Brazil international joined Bayern on a season-long deal from Barcelona in August, after being deemed surplus to requirements for the 2019-20 season by Camp Nou boss Ernesto Valverde.
Much was expected of Coutinho when he completed a £142 million ($182m) switch to Barca from Liverpool in January 2018, but he has struggled to stand out in a side full of world-class talent.
The 27-year-old has been handed the chance to reignite his career in Germany and has already emerged as a regular in Niko Kovac's starting line-up.
Coutinho has contributed two goals and three assists in eight appearances across all competitions for Bayern this term , but former Liverpool midfielder Hamann has been unimpressed by his start to life in the Bundesliga.
"For me, he is a stranger in the team," he told Sport Bild . "I cannot remember a good situation [he's produced] in attack.
"If you have such a player in the team and he does not show any special moments, then, of course, you get problems because in the defence he only counts as half a player.
"He has 30 or 35 games left so he has to start sometime [to show good performances]. He is still on loan from Barca until the end of the season."
Coutinho took in another full 90 minutes of action as Bayern secured a narrow 3-2 win at Olympiacos on Tuesday night.
The mercurial midfielder was unable to affect the Champions League clash in any meaningful way, but Kovac defended his performance during a post-match press conference.
"At the moment he lacks a little bit of luck," said the Bayern head coach. "He always has good actions. I am very satisfied with his performance as well as with the performance of the others."
Coutinho will have another chance to prove himself when the German champions welcome Union Berlin to the Allianz Arena on Saturday afternoon.
Bayern are currently sitting third in the Bundesliga, but they could return to the summit if results go their way this weekend, with current leaders Borussia Monchengladbach due to take in a tough trip to Bayer Leverkusen.