This move highlights the problem of not capturing the knight earlier. If you play similar positions with Black, always watch out when White castles and gets rid of the knight pin. If you forget about it, you might find yourself with a trapped bishop.
1. d2d4  1... g8f6  2. c2c4  2... e7e6  3. b1c3  3... f8b4  4. e2e3  4... b7b6 Black has many options in this classic Nimzo-Indian line. 5. f1d3  5... c8b7  6. g1f3  6... e8g8  7. e1g1  7... c7c5 