Barca's ability to make any signings mid-season will be dependent on them being able to free up some money, so they are scouring the market for players that could arrive on loan at first.
Sources told ESPN the club will target players that are not playing regularly elsewhere. They are prepared to include an option to make any deal permanent but don't want it to be obligatory.
Barca explored the option of signing Sterling last summer and would be keen to revisit the option in January. However, the cost of any operation and City's reluctance to let the England forward leave without an obligation for Barca to buy next summer makes any deal complicated.
Ziyech and Werner, therefore, have emerged as alternatives. The competition for places at Chelsea has seen them pushed to the fringes of Thomas Tuchel's plans. Sources have told ESPN that Barca have asked about the availability of both players.
Ziyech has started just two Premier League games this season, although he has made 11 appearances in all competitions. Werner, meanwhile, has been limited to nine appearances in all competitions, with only six of them coming as a starter.
ESPN have previously reported that Barca are also monitoring Cesar Azpilicueta's situation at Stamford Bridge. The defender is out of contract next summer and new coach Xavi Hernandez is interested in bringing him in on a free transfer.
Sources told ESPN that Xavi's immediate priority, though, is to add width to his attack, but the club's attempts to make any signings are being limited by their financial situation.
Director of football Mateu Alemany confirmed last week that, as things stand, Barca will be unable to register any new signings when the transfer window opens in the new year.
Barca's cap, which includes player wages and any signings throughout the season, has been fixed at €97m. However, they can spend 25% of any money they save in salaries or make on player exits.
The club have other ways to increase their spending limit, such as bringing in new sponsors, but sources told ESPN the club are primarily focused on moving on players who are not part of Xavi's plans to make space on the wage bill.