The Spanish international has rarely featured for the Blues this season, which in turn resulted in speculation that linked him with moves to Manchester United, Valencia, Atletico Madrid and Paris Saint-Germain.
However, the 25-year-old has conceded that his preparations for the ongoing campaign were disrupted by an injury.
"We flew to the US, where we were training and playing a friendly tournament, a common practice in pre-season. After a few days and a couple of games I felt a bit of pain on my quadriceps and I had to slow down," he wrote in a blog.
"It was something new for me because I had never had these kind of problems, but I kept calm. It is still the beginning of the season. That's why the first weeks of training were a bit tricky for me."
Mata signed for Chelsea from Valencia in 2011.