Ashton, who is executive producer of 'The Beautiful Life', revealed Mischa has been a true professional on the set following her recent hospitalisation.
He said: "She is amazing in the show. I can't wait for people to see it. I think people will be really impressed. For me, I think it is the best I have ever seen Mischa act. I was so excited to have her on the show. I was already a fan of her work, so it was great to get her.
"I have seen the first episode and she is unbelievable."
Ashton has also admitted he had no idea why Mischa was placed under involuntary psychiatric hold at Los Angeles' Cedars-Sinai Medical Center on July 15.
He told gossip website RadarOnline: "I didn't know what was going on. I don't think anyone really knew what was happening. I am a producer of the show and so I had to do everything I could to protect the show. I spoke to Mischa about three days later and I understood what was going on. We're fine, and most importantly she's fine - she's great."
Mischa's co-star Sara Paxton has revealed the cast and crew showed their support for the troubled 23-year-old actress when she arrived at work just two days after being released from hospital.
She said: "We are all really close. We hang out together and we go and watch movies together and have dinner together. We are very good friends.
"When we saw Mischa we all had a group hug.
"She is awesome. We are all working, and moving forward. Everything is great. Everything's totally back to normal."
'A Beautiful Life' - in which Mischa plays a model who is desperate to remain successful - is due to air in the US next month.
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