On-loan Blackpool striker Euell, who scored the first goal by a Rovers player at the Keepmoat Stadium since January 22, cancelled out Danny Graham's 17th league goal of the season.
John Eustace was sent off 10 minutes from the end, after collecting a second yellow card, But both sides lacked a cutting edge to claim victory.
The draw dents Watford's play-off push while Rovers are still without a win in six league games as they look to move clear of the npower Championship relegation zone.
Doncaster started the livelier of the two teams with Scott Loach having to tip Sam Hird's shot over the crossbar.
The home side almost opened the scoring in the 14th minute when Hird sent in a low ball towards Euell, and after the on-loan forward found Billy Sharp, the fit-again striker just flashed his shot wide of the post.
The visitors hit back and Graham smashed a shot from distance, which was just deflected wide of Gary Woods' goal.
The Hornets broke the deadlock after 22 minutes when Andreas Weimann cut the ball back towards Graham, who headed in from close range.
Watford threatened to extend their lead 10 minutes later but Stephen McGinn, under pressure from Joe Mills, drilled his right-footed shot over the crossbar.
Five minutes before the interval, Euell and Sharp again combined well but Sharp's strike fizzed past the post.
Rovers equalised on the stroke of half-time when Hird sent Euell through and he fired a shot past the keeper, with the ball hitting both posts before crossing the line.
After coming on as a substitute, Marvin Sordell almost made an immediate impact but his shot went over the crossbar.
Euell was making a nuisance of himself up front and shot wide after Loach parried his initial effort back into his path, while Sharp had another effort he failed to control.
Euell could have had his second goal of the game but his shot just dipped over Loach's bar.
The Hornets were reduced to 10 men with 10 minutes remaining when Eustace, who was booked in the first half, was shown his second yellow card for a deliberate handball.
Doncaster tried to make their extra man count as Paul Keegan's volley was deflected just wide, while Brian Stock blasted over.
The impressive Euell had a final chance to win the game, but he headed Hird's cross wide of the goal.