Fellaini came up against Jagielka during England's 1-0 friendly victory over Belgium at Wembley on Saturday and caught the eye with a bustling performance in midfield.
The 24-year-old is part of a highly-rated, young Belgium set-up that includes new Chelsea signing Eden Hazard, Tottenham target Jan Vertonghen and a number of other players attracting a lot of attention from across Europe.
Fellaini has really progressed at Everton since his move from Standard Liege in 2008, so much so that Jagielka can see the midfielder becoming a target for a number of clubs.
Jagielka said: "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. Unfortunately, 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.
"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."
Jagielka added in the Daily Mail: "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."
Belgian newspaper Le Soir last week suggested Fellaini may be on the radar of European champions Chelsea, but when asked about the rumours, the midfielder responded: "I am not aware of any of this."