Madrid have sacked two head coaches in 2018-19 – Julen Lopetegui and Santiago Solari – with the club third and 12 points adrift of Barca in the league, while their European dominance was sensationally ended by Ajax.
Ronaldo, meanwhile, scored a hat-trick to lead Juve past Atletico Madrid and into the Champions League quarter-finals.
Speaking about Ronaldo, Semedo believes his absence has been felt to a considerable degree by both Madrid and La Liga.
"Real Madrid have noticed his absence a lot," Semedo told Portuguese TV channel SIC.
"I cannot say that it was bad for us that Cristiano left. With him it would have been a more difficult league for us, more competitive.
"He [Ronaldo] is a player that requires a lot of attention and care. Without Lionel Messi we would play well, but not the same."
Messi has scored a La Liga-high 26 goals this season and 36 in all competitions, while Juve's Ronaldo has netted 24 for the Italian champions.
Both Semedo and Ronaldo were named in Portugal's latest squad with the latter returning to the fold for the first time since the 2018 World Cup.
Despite impressing since arriving in Turin, Ronaldo's arrival did have teammate Douglas Costa initially questioning the impact he'd have.
"Believe it or not but when Cristiano Ronaldo's arrival was being talked about, I asked 'what will the dressing room be like?'" Costa told YouTube channel De Sola.
"We all have to love each other as team-mates because we're all looking for a place in the team - but Cristiano won everyone over very quickly."