Rahul Dravid's side have a tough task ahead to qualify for the playoffs after losing three matches in a row, including a last ball thriller in Delhi.
Simons said Watson, who will be available after the series against West Indies, would surely make a huge difference.
"All the Australian cricketers are good and would be important for their teams. They contribute off the field too. David Warner with his striking capabilities will make us stronger and Michael Clarke as a pure batsman for Pune Warriors but for Rajasthan Royals, Watson would make a massive difference with his all-round capabilities," Simons said.
"He can bat and bowl and in fact is as good as two cricketers. He also brings with him the experience, confidence and the information which would be useful for the team." he added.
Meanwhile, Simmons said Australian opener David Warner will be joining his Delhi teammates on May 7.
"We will have to wait for Warner as he has flown back to his country as he wanted a break to be with his family. He would be playing for us on May 7 against Kolkata Knight Riders. But I am not sure we could have included him for this match even if he was available," he said.
Regarding the thriller against Rajasthan Royals on Sunday, Simmons said, "It was a very close affair. In fact we did not win it but the Rajasthan Royals lost it. Like what Dravid said that they should have finished it earlier. Things should not be allowed to a point where one good over, a four here or a wicket there can make all the difference."
Simmons praised the Royals sensational opener Ajinkya Rahane, saying he had a good idea about the youngsters brilliant talent.
"It was his fifty against England when he played for India and there I felt he had special talent. He has the heart and in Dravid's company he has grown in confidence. He hit a marvellous six after Morkel had turned the tide.
"He has good technique, sound mind and most important is the character. In the nine matches he has played, in this IPL he has shown a lot of character. He has played a lot of inside out shots over extra cover. We analyse and work out strategies for such things before the match and would do so this time too," he said.
Simmons was also satisfied about his team's performance in this season so far.
"We have played quite consistently and have a strong side. I don't feel there are any particular areas we have to improve on. It is all about momentum. We could have 20 more runs on the board. We will have to win one – two more games for the playoffs.
"I feel CSK is the most balanced side but of late they have not played up to their potential. Kolkata Knight Riders and Royal Challengers all have match winners in their ranks," he said.