The world renowned physician spent two years with the club from 2010 to 2012 and dealt with the Spanish striker when he suffered knee problems.
Brukner told talkSPORT: "His injuries were reasonably serious. He had some knee problems. He had a number of arthroscopies in his knee and certainly his knee is not 100%.
"How much that is affecting his football is hard to say. On occasions you see some excellent play from Fernando.
"I think the timing was unfortunate. It happened at the end of the season just before the 2010 World Cup. He was obviously desperate to play at the World Cup and probably pushed things a little too hard.
"When he joined the Spanish team He probably got a bit overenthusiastic and pushed too hard too early, had a setback, then had a very disappointing World Cup even though he's got a World Cup winners medal.
"He came back to Liverpool and wasn't happy at Liverpool. He was very frustrated and basically I think he lost a lot of confidence. Whether that was partly related to his knee, whether he didn't have 100% confidence in his knee, [I don't know].
"The testing we did at Liverpool showed he was still the fastest player at the club when he was there. So I think a lot of it is confidence, confidence in his physical ability to do things and also confidence in his ability to do things mentally.
"Fernando Torres had had a dream life. He'd had a dream career. He'd never had a setback. Everything had gone so well.
"He'd gone from Atletico [Madrid] to Liverpool. He was a sensation when he came to Liverpool. It's all about how you cope with setbacks and he had these injury setbacks and maybe some elite sports people aren't conditioned to deal with them.
"The rest of us are used to setbacks and have them every day and maybe we get better at coping with setbacks than elite players."