The Spain international has been plagued by hamstring and pelvic injuries since joining from Atletico Madrid for Â£32million but that has not stopped him scoring 10 goals already, including the winner in the 2-1 victory against Liverpool on Saturday.
Costa has been left out of the national squad for a Euro 2016 qualifier against Belarus next week and a friendly against Germany three days later.
By the time the league campaign resumes at home to West Brom on November 22, Mourinho expects his star man to be firing on all cylinders.
"I think Spain made a fantastic decision not just for us but for them," he said.
"He has 15 days to get to the top of his game and when they need him they will get a good Diego not a bad one.
"Maybe the injury was good because it stopped him
When he became injured he had time to recover, step-by-step, and at this moment he is in a good condition."
Mourinho also hopes the international break will give midfielders Cesc Fabregas and Ramires time to recover from hamstring and abductor problems respectively after risking them at Anfield.
"I need to give credit to them - they both played injured, risked everything to play," he added.
"Fabregas gambled and I gambled
He can join up with Spain but he will be back (as he is not fit)."
Unbeaten Chelsea maintained their four-point lead at the top of the table with a ninth win in 11 matches and stretched a remarkable 15 points ahead of Liverpool, runners-up last season.
The Reds were not realistically expected to contend this campaign but even without them in the running, Mourinho knows there is plenty to be done before his team can even entertain thoughts of the title.
"If this was the Spanish, Portuguese or German league where you have two teams competing to be champion, if you leave one team 15 points behind you are almost there but this is England and every match is difficult," he added.
"It is not about Liverpool, there is Man City, Arsenal, Tottenham, Southampton.
"We are in a good situation, we have already played the four matches in Liverpool and Manchester, four very difficult matches away, but it is the beginning of November so there is a long way to go."
If Chelsea have a long way to go, Liverpool's task is even more onerous as a fifth defeat of the season - and third in a week after reverses at Newcastle and Real Madrid - leaves manager Brendan Rodgers with some difficult questions to ask himself and his players.
The decision to rest key players in Madrid, making seven changes, did not pay off in terms of a result against Chelsea.
"Obviously there is pressure on when you don't get the result but I can only refocus now and it is important we get back to winning ways after the international break," said the Reds boss.
"It is a difficult moment for us as we're not getting the results and it has been a tough week really.
"Our focus is getting into that top four with the considerable change we have and we still have a big possibility to do that."
With little goal threat to trouble the leaders, the return of striker Daniel Sturridge, who has not been seen in a red shirt since August 31 because of thigh and calf injuries, cannot come soon enough.
He is expected to be ready for the trip to Crystal Palace on November 23.
"The break comes at a good time for us
We are hoping to get one or two injuries back after that and then we have to look to get on a run," added Rodgers.
"It has been a long time now (without Sturridge)
We are hoping he can now get to work because he hasn't trained with the team at all in the time off.
"Hopefully over the course of the next couple of weeks he can be out there training and we will have him back, which will be important as he has had a big impact for us."
Emre Can's first goal for the club since his Â£10million summer move from Bayer Leverkusen gave Liverpool the ideal start - with the aid of a huge deflection off Gary Cahill but the Chelsea defender got himself back in credit by bundling the ball over at a corner for the equaliser, which required the assistance of the goal decision system.
Costa displayed his clinical best with a sweet first-time strike to drill home Cesar Azpilicueta's cross, although Liverpool had a fair shout for a penalty ignored late on when the ball appeared to strike Cahill's arm.
Source : PA