The Spaniard, winner of seven Grand Tours, planned on continuing through 2018 but changed his mind in late July after a below-par Tour de France.
"Alberto is irreplaceable considering both his athletic talents and popularity," general manager Luca Guercilena told Cycling Weekly.
"Alberto originally wanted to continue to 2018 and we wanted to keep him. At the point he decided to quit, the other athletes were already committed to other teams for 2018.
"It's clear that we were trying to negotiate with Fabio Aru and we wanted to try with Mikel Landa, but they already had agreements. The reality was that no one was on the market at the time that we went to look."
Aru agreed to sign with UAE Emirates, which should be announced soon, and Landa is leaving Sky for Movistar.
The team also considered Rigoberto Urán and Daniel Martin – the former renewed with Cannondale and Martin is joining UAE-Emirates.
Trek-Segafredo sports director Steven de Jongh said in a separate interview that Aru and the others were asking too much money. Aru's offer, he said, was too rich for Trek-Segafredo's blood despite the team wanting an Italian leader for their co-sponsor Segafredo.
"A rider like Canadian Michael Woods? It's one thing to take a rider who is developing and maybe in two years he will have a chance in a Grand Tour given how great he went in the Vuelta a España, but he still doesn't offer a guarantee performance in the Tour de France," Guercilena added.
"We already have Bauke Mollema and Gianluca Brambilla [joining from Quick-Step]. When you sign this type of rider you want someone who'll be able to provide some certainties of doing something important in the Tour de France."
The team's focus will change slightly in stages races. In smaller stage races like the Volta a Catalunya or Tirreno-Adriatico, they will allow others like Jarlinson Pantano to race for stages.
"We invested in young athletes starting already two years ago, to help them grow. I'm talking about Giacomo Nizzolo, Fabio Feline and Bauke Mollema and Jarlinson Pantano, who was a helper all year in 2017," Guercilena continued.
"Bauke will return to be the captain for the Tour de France. He did very well two years ago in the Tour de France and this year he won a stage. I think that he can still improve a bit.
"Gianluca Brambilla will need to transform himself to more to a stage racer especially for the Giro d'Italia, where he already wore the pink jersey. We want to move in that direction with him. We are faithful in the men to have good season."
The team will continue to work with John Degenkolb for the Classics and sprints. He won Milan-San Remo and Paris-Roubaix in 2015 but has yet to reach full speed after a training incident in January 2016 with his former team Giant-Alpecin.
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