The Belgium international enjoyed a prolific loan spell at The Hawthorns in the 2012-13 season, netting 17 goals in 35 league appearances.
On Sunday, Man United could hammer another nail in the Baggies' Premier League coffin, given their 12-point gap from safety, when the two lock horns at Old Trafford.
"It's a painful one for me, because a few of those guys are my friends, so it's hard for me to see them in that position," Lukaku told MUTV.
"But sometimes it's good to take a step back and then bounce and learn from it and try and improve on it.
"I just hope they don't go down and that they can learn from the mistakes that they made in the pre-season or whatever.
"I don't know what their problem has been this season, because you only watch the highlights, so you don't know what's happening in their game, but I just hope they find the key to success for the Premier League."
Lukaku has gone from strength to strength following his stint at West Brom, enjoying a goal-laden spell with Everton before joining Man United last summer.