According to the official club Twitter feed, the player was bleeding and also booked before he left the field of play:
Ossie is down and bleeding after tangling with Sagna. He’s also been booked for the incident.— Everton (@Everton) April 6, 2014
As with any blood injury, the player must leave the field before being allowed back on but speculation was that Osman's afternoon was over:
Ossie is off down the tunnel, is his afternoon over already?— Everton (@Everton) April 6, 2014
The speculation was proved to be correct with Ross Barkley coming on for Osman after just nine minutes.
Barkley is on for Osman after just nine minutes. #EFCAFC— Everton (@Everton) April 6, 2014
It's hardly the way the midfielder would've wanted his day to pan out after being made captain on the day that his testimonial was announced, via the Liverpool Echo:
And it will be a blow for Roberto Martinez to lose one of his most experienced players, and one who has provided consistency in the midfield areas for the Toffees this season, per WhoScored.
There are just a few games left of this Premier League season and Everton are harbouring genuine hopes of catching today's opponents Arsenal who currently hold fourth position.
A Champions League spot is certainly not beyond Martinez's side, but he will need the likes of Leon Osman restored quickly to the starting line-up to help him achieve Everton's highest league placing since the 2004/05 season.