Graham was hurt on his only carry in Sunday's 19-13 win, limping off after landing awkwardly when he was tackled after a 3-yard gain.
"His status for the rest of the season is very much in question right now," Gruden said Monday. "I don't have the exact medical terms, but I will say right now, it doesn't look like he'll play anytime soon."
Warrick Dunn took over as the featured back when Graham left, finishing with 53 yards on 20 carries to move ahead of Ricky Watters for 19th place on the NFL's career rushing list with 10,657 yards. He also had four receptions for 65 yards against the Vikings.
Graham's injury also figures to accelerate Carnell "Cadillac" Williams' comeback from an injury that kept him from playing since tearing the patellar tendon in his right knee in September 2007.
"I am ready to go," Williams told the Tampa Tribune. "I have put in a lot of work. I feel confident. I'm mentally ready and physically ready. I just hate the situation had to come like this with E.G. going down, but that's football. I'll make the best of the opportunity."
The fourth-year pro was moved from the physically unable to perform list to the 53-man roster last week, but he was inactive Sunday.
"Obviously we brought Cadillac up with the understanding he's ready to go," Gruden said. "We're very disappointed for Earnest. He means a lot to us. He's done so much for us here in the last couple of years. He a key reason why we are where we are."
Graham started 10 games in 2007 and rushed for 898 yards and 10 touchdowns in helping the Bucs win the NFC South.
Information from The Associated Press was used in this report.