Lukaku will be looking to continue his impressive form during Saturday evening's Barclays Premier League derby with Aston Villa at the Hawthorns.
The 19-year-old has already indicated he would like to remain at Albion for a second season and has impressed Foster with his desire and hunger to learn.
Foster said: "This last month, it's like the penny's dropped with Rom and he has been superb.
"It was probably the inexperience, about getting used to playing Premier League football week in and week out, knowing what you are going to be coming up against, knowing your role as a striker.
"He's found that. He uses his strength to hold players off and he's starting to get into the box to get on the end of things. He's got everything.
"He will be ready for Chelsea when he goes back, without doubt, whether that is next season or whenever.
"Chelsea paid so much money for him, and they know what they've got now, and I'm sure they're very happy with him.
"But Rom just looks settled and enjoying his football so I don't see any reason why he wouldn't want to come back here."
Foster is relieved Chelsea opted not to recall Lukaku during the current transfer window.
He said: "I started hearing a few whispers that there was a possibility of it happening especially with the way Chelsea were going and with Torres misfiring.
"It screamed out for him to go back. I think it would have been a huge loss for us. We would have been gutted to lose him.
"He is not only a good asset on the pitch. He is such a good lad to have around the place. He's very level-headed, has no ego to him and is always the last one off the training pitch."