"He's been absolutely fantastic. He's a terrific guy and I'm delighted with him. It's just a loan the way it stands but you'd never think the boy was only on loan the way he's settled in and the way he's played," Lambert told the Birmingham Mail.
"Whatever Chelsea price him at, that's their prerogative but we're thankful they gave us that opportunity to let him come here."
Bertrand has started three games for the Villans.