Spaniard Alonso has been a near ever-present fixture at left wing-back under manager Antonio Conte, though missed the shock Premier League thumping by Watford in addition to the victory over West Bromwich Albion and the FA Cup fifth-round demolition of Championship strugglers Hull City due to an initial hamstring concern and a subsequent ankle knock.
Another hamstring problem has ruled Barkley out of the same trio of matches, with the attacking midfielder limited to just one start and two substitute cameos since joining the defending champions from Everton in a £15m ($20.9m) deal during the early stages of the recent January transfer window.
Manager Antonio Conte confirmed after the victory over Hull that Alonso would be "ready" to play against Barcelona at Stamford Bridge and both he and Barkley took part in a session at Cobham on Monday [19 February] morning.
Meanwhile, Eden Hazard, N'Golo Kante, Cesar Azpilicueta, Victor Moses and Thibaut Courtois should all return after being rested in advance of that high-profile meeting with the La Liga leaders.
"Marcos Alonso and Ross Barkley were part of the group which trained at Cobham this morning ahead of tomorrow's Champions League game against Barcelona," Chelsea confirmed via their official website.
"Both players have missed our last three matches but they joined their team-mates for this morning's session, of which the first 15 minutes were open to the world's media."
While Alonso and Barkley should be available, however, David Luiz and Tiemoue Bakayoko - both absent since the defeat to Watford in which the latter was sent off after a nightmare opening half an hour - are expected to miss out after respective unspecified knocks prevented them from training. Conte had already revealed that the latter was unlikely to be in a position to feature.
"Having missed our last couple of games, David Luiz didn't train," the Chelsea boss added. "Tiemoue Bakayoko, who was unavailable for Friday's FA Cup win over Hull City, was also absent from the session."
Barcelona, who equalled their best-ever La Liga unbeaten run with a 2-0 win at Eibar on Saturday [17 February] thanks to goals from Luis Suarez and Jordi Alba, will definitely be without Philippe Coutinho for their trip to west London with the former Liverpool playmaker cup-tied.
Right-back Nelson Semedo is suspended, while injury-prone defender Thomas Vermaelen has travelled but is unlikely to play following his own hamstring issue.
Unlike his Chelsea counterpart, head coach Ernesto Valverde resisted the urge to rest any of his key players in the Basque Country with the Catalan giants eager to get back to winning ways after successive league draws against Espanyol and Getafe had trimmed their lead over nearest challengers Atletico Madrid to seven points.