Christian Eriksen is in "perfect" condition but Thomas Frank thinks he will only be ready to make his Brentford debut in "a few weeks".
Brentford signed Eriksen on a deal until the end of the season following his release from Inter in December, which came after he suffered a cardiac arrest while representing Denmark at Euro 2020 last year.
The 29-year-old was fitted with an implantable cardioverter-defibrillator (ICD) device, which made him ineligible for professional football in Italy.
Eriksen has been training with his former club Ajax and Frank has no concerns about his fitness, though he will not join up with his team-mates until after Saturday's FA Cup trip to Everton.
"It is a little bit of a miracle of course that first and foremost he is healthy but also that he wants and feels ready to play football again," said the Brentford boss, who was self-isolating following a positive COVID-19 test.
"I would say no [concerns]. Of course, I am not a doctor but all the specialists who have seen him, met him and tested him know that everything is perfect. I know Christian and his family would never take the slightest risk in that sense that something could happen when he is training or playing a football match.
"For me it is down to the mental part of it and getting back into rhythm, playing football at a top, top level and managing it, which I expect will be fine.
"Christian will arrive on Sunday and train with the team on Monday. That will be his first training session. It is hard to say when he is available. I will know a lot more after Monday.
"I hope weeks, a few weeks, but I don't know. I am much more clever when I have seen him train."
Frank heralded Eriksen as the "greatest ever signing" for Brentford, who have slipped down the Premier League table after suffering six defeats in their past seven outings.
"We got it over the line so that was fantastic. It’s potentially the greatest ever signing for the club. I spoke to a fan last night and he mentioned a guy from 70 years ago that was at the same level. I think this is still the biggest signing," said Frank.
"The story behind it… I think it’s going to be an unbelievable day, the day Christian steps back on to the pitch. All of you guys have seen him for years forming as one of the best midfielders in the Premier League and what happened to him in June was crazy.
"It was a shock for all of us, so to see him out there on the pitch soon, it's going to be a big day."