The Toffees’ record signing has continually improved since his big-money move in 2008, with manager David Moyes recently suggesting that an advanced role behind the striker could be the future for the 24-year-old.
And Jagielka was again impressed with the influential midfielder’s most recent performance, in England's 1-0 victory on Saturday over Belgium, and has conceded that it would be hard for the club, with their tight financial restraints, to hang onto the player if a suitable bid was made.
"With the way the club is, if somebody comes in with a £20 million-plus offer, the chances are he's going to be sold" - Phil Jagielka
He told the Daily Mail: "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 Fellaini 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 £16 million and I don't think the club would take a loss."
Fellaini was signed from Standard Liege four years ago and played a crucial role in the Toffees’ seventh-place finish last season, starting 31 matches and displaying his versatility with starting berths in a holding or attacking midfield position.
Jagielka commented: "He's developed in that time, especially in the last few games of the season, [playing] just behind the front one, and he's destroyed teams.
"In terms of marking Fellaini, he's faster than I remember. He must have put a yard on.
"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."