Watkins played nearly 150 games for the Bees with the majority of his time spent under Frank. Speaking to the club'd website, Frank praised Watkins and hopes he can grab a goal soon though not against Brentford on Wednesday night.
"Ollie is a top player. He has developed well at Aston Villa over the last few years, where he is constantly adding layers," said the Bees boss.
"He will score goals again - just hopefully not tomorrow! He can even be the top goalscorer in the league, as long as he doesn't score against us."
Before making the move to Villa, Watkins was a standout striker under Frank.
"He was very important for us. He was here for three years: two years as a winger and then we moved him to a striker, where he really flourished.
"I really liked him as a striker and he played a big part in that very good season. We played some fantastic football but we just narrowly missed out on promotion.
"But he helped get us up to that position, along with the other players, and then we took the final step. He's been a big part of the journey we've been on."
- Get breaking football news faster with the new Tribal Football app! Download now on iOS and Android to stay ahead of all the latest transfers and football updates. Download now: App Store, Google Play