Rajasthan have lost three matches in a row, including last night's thriller in Delhi, and their job to qualify for the play-offs has become quite tough.
Simmons felt Watson, who is coming to IPL-5 after their series against West Indies, would surely make his presence felt.
"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," Simmons 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."
Simmons also informed it is still time that Warner joins them.
"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.
Talking about the narrow victory over the Rajasthan last night, 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." More PTI DS AT