Sunderland and West Ham had been set to battle it out for the 23-year-old's signature this summer, but with reports today suggesting that West Ham are now pursuing alternative targets, the Black Cats are now seen as frontrunners to sign Alonso.
The Spaniard spent the second half of last season on loan at the Stadium of Light and played a key role in Sunderland's great escape and journey to the Capital One Cup final under the stewardship of Gus Poyet.
Alonso is thought to be available for around £1.5m and after making such an impact on Wearside last term, the defender has attracted attention from a number of possible suitors with West Ham seen as the biggest threat to Sunderland's chances.
Poyet is desperate to bolster his defensive ranks in time for next season with the club particularly short in the left-back department following the release of both Andrea Dossena and Phil Bardsley, and the sale of versatile utility man Jack Colback.
The Uruguayan manager recently told Spanish publication MARCA that he is doing everything within his power to bring Alonso to Sunderland on a permanent basis, saying;
"I'm there with 'Marquitos' Alonso, I already had him for six months. We are doing everything possible so that Fiorentina give him to us."
Modibo Diakite made nine appearances for Fiorentina during a loan spell last season and Sunderland are expected to make a cash plus player offer for Alonso in the coming days with Diakite going the other way, making his return to Serie A permanent.
If the reports today regarding West Ham's interest in Insúa carries any weight, then Sunderland are surely the most likely to land the Serie A defender. But Fiorentina are yet to comment on the future of their player, and are thought to be keen themselves on having a closer look at Alonso during pre-season before they make a decision on the Spaniard's future.