He would be a downgrade on Lukaku. Had a great season last year and knows where the goal is but overall he's definitely a level below Lukaku.
Lukaku is poor in play build up, poor in one touch passing and dribblling, he's very comfortable when he manage to control the ball in the wing and run away with it, he needs lands of space in front of him to really shine. That's why he underperformed in Chelsea and with us and was a star at Everton and West brom.
Ben Yedder has comparable goal scoring records to Lukaku, and proven European scorer who scores and play with both feet, and is more capable on and off the ball, able to dribble, fantastic at link up and one touch play. And swapping Lukako for him would free approximately 40 millions for other positions.
But sure, he's a downgrade!
If we manage to get Ben Yedder that would be a great signing. It will have a Berba effect. Somebody who is actually capable to hold the ball on his feet, shake the defenders off and make a decent pass to our pacy players, and is also capable of scoring 20+ goals.
He will replace what we miss in both Sanchez and Lukaku. Maybe not Sanchez pace or Lukaku's strength, but definitely Sanchez' build up and more than Lukaku's goals.