The Austrian believes Chelsea have made a "fantastic signing" but is eager for Werner's barren run in the league to continue for the time being.

"I know this player very well. I had two fantastic years with him. We brought him in from Stuttgart for small money, if you want, because we knew the game we played in Leipzig was perfect for him," said Hasenhuttl.

"The first year was amazing. He scored massive goals, I think it was 24 in the first season in the Bundesliga and we were vice-champions (runners-up).

"It was a fantastic year, we both enjoyed it a lot. I have worked with a lot of players in my entire managing career – he is definitely one of the quickest I've ever worked with.

"He is really hard to defend, he has a fantastic finish. In the second year, it delivered more problems with some injuries.

"He is getting a more complete striker now, a better package. He learned a lot last season and is a fantastic signing for Chelsea.

"He's only happy when he scores. Hopefully he's unhappy after the game – we will do everything to keep him unhappy."

Werner was on target in midweek when Germany drew 3-3 with Switzerland in the Nations League and also found the net during Chelsea's Carabao Cup tie with Tottenham last month.

He arrived during a busy transfer window for the Blues, which also included the signings of goalkeeper Edouard Mendy, defenders Ben Chilwell and Thiago Silva, midfielder Hakim Ziyech, and compatriot Kai Havertz.

Hasenhuttl feels Chelsea boss Frank Lampard has recruited an enviable array of talent and admits Saints face a more difficult task than last season when they won 2-0 in west London on Boxing Day having lost the home fixture 4-1.

"If they wouldn't be a better team, they would have spent the money for nothing," he said.

"Definitely they have to be a better team than last season because with Timo Werner, Kai Havertz and a few other signings they have so much quality now – not only in the attacking positions but mainly there – so normally a team that is prepared to win titles and trophies.

"And in the end for us more difficult than last season, maybe.

"We had two different games last season, the first we lost, the second we played fantastically. The level is high. We must reach this level because otherwise it will be difficult to get something."