Despite contributing seven goals and eight assists in the Bundesliga and Champions League this season, it has been claimed that Coutinho will return to parent club Barcelona in the summer.
However, despite the cost of any potential transfer, Rummenigge has encouraged the playmaker to impress in a bid to secure a longer stay in Germany.
Rummenigge told Bild: "Why is it [the transfer] not decisive? This is a difficult question. I think he played well in some games while in others he gave the impression of being a little inhibited.
"I don't know why. The fact is that we all believe he is an exceptional footballer, we see him in training and sometimes during competitions.
"Everyone tries to help him, the coach, his teammates. Now we have to hope that in the crucial weeks starting now, he will become an important factor in helping us win titles."
Coutinho is expected to feature when Bayern face Chelsea in the Champions League on Tuesday night.