Jagielka was impressed with the performance of Fellaini when in action for England against Belgium in Saturday's friendly international at Wembley. The central defender knows how crucial Fellaini is to the Toffees but is realistic about their 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."
Jagielka added: "We bought him for £16 million 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.
"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."