Success was sent out to the La Liga outfit to get some more game time under his belt but there have been suggestions in the Spanish media that the Nigerian international is overweight.
Injuries have plagued Success for much of the campaign so far but Lopez has refused to say the forward is carrying too much weight and has instead stated the game against Atleti simply did not suit the 22-year-old.
He told AS: “I am the main endorsement of Isaac coming here and I think, for this game, the minutes he will play, he may not be the right player.”
Uwe Rosler was sacked by Fleetwood following a 3-0 reverse at Doncaster which condemned Town to a seventh straight defeat in all competitions.
Cambridge interim head coach Joe Dunne was pleased with how his side adapted to beat Grimsby 3-1.