The 48-year-old Madrid-born coach replaces Claudio Barragán, who was dismissed ten days ago, and will be formally presented on Thursday morning at the Nuevo Pepico Amat before making his bench debut this weekend against Eibar. Calleja arrives at Eldense with only a couple of training sessions to prepare his new squad.
Eldense's takeover by Messi is not yet fully complete, but the Argentina great has reached a preliminary agreement to buy out the club's Colombian ownership group, led by Juan Carlos Trujillo, and become majority shareholder — subject to approval from Spain's Consejo Superior de Deportes.
It would be his second investment in Spanish football after taking control of Segunda Federación side UE Cornellà earlier this year. The Eldense job represents a significant opportunity for Calleja, who arrives with considerable experience across two divisions and takes charge of a club whose century-long history has never previously included a spell in LaLiga.
The coach has taken charge of 129 matches in Primera División, split between Villarreal and Deportivo Alavés, as well as 90 in the Segunda División with Levante and Real Oviedo, and a further 18 in the Europa League.
His best work came at Villarreal, where he twice led the club to a fifth-place league finish and European qualification across two separate spells in charge, before also keeping Alavés up following a mid-season appointment and guiding Levante to the brink of promotion.
Eldense currently sit in the Segunda División, having secured promotion back to the second tier, and face an immediate examination of their form against an Eibar side that visits the Nuevo Pepico Amat this Saturday.
Barragán's exit followed a difficult run of results that left the club needing a swift change of direction just as attention off the pitch turns increasingly toward Messi's incoming ownership.