The 23-year-old Portugal midfielder is Chelsea's fourth recruit under new manager Jose Mourinho.
A first half Tiago header set up Atletico Madrid's 2-0 victory over Espanyol as they seek to get back on track following a stuttering start to the La Liga season.
Atletico Madrid put last week's painful defeat by Real Madrid behind them with a 3-0 win over Osasuna on Sunday that included two goals in the space of a minute and allowed them to return to the top four in La Liga.
Vietto slotted the ball home with seven minutes to go after Karim Benzema had given visitors Real the lead early on.
Bayern Munich are desperate to avoid losing a third consecutive Champions League semi-final to Spanish opponents when they host Atletico Madrid on Tuesday but coach Pep Guardiola has his work cut out to end his time in Germany on a high.
Last season's treble winners Barca are in their worst run of form in La Liga since coach Luis Enrique took charge in August 2014.
Monaco owe most of their recent success to their young guns, but when they face Manchester City looking to overturn a two-goal deficit in their Champions League last-16 tie on Wednesday, they will turn to a more experienced player in Radamel Falcao.
At Rediff.com, we would like to salute their undying spirit...
Atletico Madrid sealed top spot in Champions League Group D after second half goals from Frenchmen Kevin Gameiro and Antoine Griezmann but Napoli were held to a 0-0 Champions League draw at home by Dynamo Kiev.
Schuerrle hits treble for the world champs; Lewandowski helps Poland thrash Georgia