If Jordan Henderson's worth 20m, Lennon's worth 30, but I doubt you'll get that much for him. If you got around the 15-20 mark I'd sell him. As for a replacement I'd reccomend one of the following.
Pavel Niakhaichyk-BATE-2m
Jonas-Newcastle-7m
Eljero Elia-HSV-15m
Eden Hazard-Lille-18m