I'd keep him and train him if he's not playing well. He's young, so his stats will improve and his value will go up. If he still plays bad when you try him again you can sell for more than he's worth now. I got a 19yr, Q30, 1.7m MC who's not performing so benched him for a 22yr, Q26, 750k player that's doing better. I'll be patient but if my youngster doesn't up his game I'll just replace him. Bought for 1.2m so nice profit anyways
Edit - if you have low cash and/or no good replacements, of course consider selling now