The 24-year-old Belgian arrived at Goodison Park in a reported £16million move from Standard Liege in 2008 and has been a key man for the Toffees.
Jagielka knows how crucial Fellaini is to the team but the financial realities of life at Everton mean he is realistic about the club's chances of keeping him.
He said: "Unfortunately, with the way the club is, if somebody comes in with a £20million-plus offer, the chances are he's going to be sold.
"We love Felli and we'd love him to stay. There's not been that much transfer talk but we know the situation we're in.
"We bought him for £16million and I don't think the club would take a loss.
"He's developed in that time, especially in the last few games of the season, just behind the front one, and he's destroyed teams."
Jagielka was impressed with the performance of Fellaini when in action for England against Belgium in Saturday's friendly international at Wembley.
Jagielka added: "In terms of marking Felli, he's faster than I remember. He must have put a yard on. I can't stop laughing when I see him and it's not just his hair.
"He's a fantastic player. Belgium have got a few players for the future and, once they kick, on they'll be a team to watch."
Jagielka has seen players leave Everton before, with Joleon Lescott a notable example having left for Manchester City in acrimonious circumstances in 2009.