The in-form striker was relegated to training alone after it emerged that he was reportedly the subject of a £30m-a-year offer from Chinese Super League side Tianjin Quanjian and became upset when the Blues were alleged to have dismissed the offer.
Costa, who sat out Chelsea's 3-0 win over Leicester, has been returned to the squad by boss Antonio Conte for Sunday's home tie with Hull, and Courtois says the player's determination to succeed with the Premier League leaders was not in doubt.
The official line from the club was that Costa had a minor back injury, with Courtois telling Geoff Shreeves in an exclusive interview with Sky Sports: "He has trained very well this week.
"Obviously it was not a nice rumour that went around that he had a fight or anything. He had an injury and couldn't train and obviously if you are not 100 per cent fit and you feel you cannot play it's logic that you don't train with the team.
"This week he could train again and he trained very well, so he might play on Sunday.
"He looks very committed, we cannot doubt that about Diego. You can see that in all games he plays."
He added: "Nobody in this team doubted it, so when the day before the game against Leicester that came out it was a bit of a surprise to the players but it didn't affect us in any way because we know what's the truth.