The Senegal man is still Newcastle's leading scorer with 13 goals.
"It is a tough one for him," Shearer said.
"He will believe he has a better chance of winning things there and at the moment he probably has.
"Having said that, he gave his all in a Newcastle shirt, played well and scored goals, so I don't think anybody can complain.
"But it is disappointing to see our top scorer leave. Hopefully, they will have a plan B and a plan C because it's left Newcastle with a huge gap that they obviously need to fill."