Two second-half goals from Billy Sharp maintained Southampton's five-point lead at the top of the Championship.
Sharp struck twice in the space of 17 minutes to increase his former club's relegation worries.
Doncaster fell behind when Sharp headed home Daniel Fox's corner before a mistake by Habib Beye allowed him to make it 2-0.
The visitors went close through Frederic Piquionne when the game was still goalless.
Doncaster, five points from safety with eight games remaining, are running out of matches to save themselves.
Southampton never looked back after Piquionne headed over a golden chance from Simon Gillett's eighth-minute cross.
The leaders quickly settled and Adam Lallana could have opened the scoring after a neat exchange with Sharp, but his effort was blocked.
The traffic was becoming one way and, after Rickie Lambert had a shout for a penalty turned down, Sharp went close with a header which went wide of the post.
The opener eventually came in the 58th minute when Sharp, a £2m signing from Doncaster in January, reacted quickest to get on the end of Fox's inviting corner.
And Sharp sealed the points with 15 minutes remaining when he capitalised on some poor defending from Beye before firing past Carl Ikeme.
Southampton manager Nigel Adkins: "Billy Sharp has scored two really good goals for us and it's a great three points to cap off a very good week.
"It was in the tea-leaves (for Sharp to score). He's worked very, very hard behind the scenes. Since he's joined us, his attitude has been top drawer.
"And the big thing is, here we're not just relying on one person to score the goals. It's 72 league goals we've scored now. We've got some exceptionally good goalscorers and strikers here."
Doncaster manager Dean Saunders said: "The ref cost us the game. He gave them a corner which was never a corner. They scored from the corner. It was a mistake for the second goal.
"It's disappointing we lost points against a top of the league side.
"I think it's typical we sold Sharp to Southampton and then he scores two goals against us. He came back to haunt us."