Jennifer Aniston "worried" about Matthew Perry on the set of 'Friends'.

The 53-year-old actress starred as Rachel Green in the iconic sitcom, and an insider has claimed that Jennifer felt particularly worried about her co-star, who struggled with addiction during his time on the show.

The source explained: "It’s fair to say he was by far the most fragile member of the cast. Jen saw that - it worried her enormously - and she made a point to check in on him whenever she could, on and off the set."

Jennifer also starred alongside Courteney Cox, Lisa Kudrow, Matt LeBlanc, and David Schwimmer on the sitcom, and the actress always thought of them as a "family".

The insider told the new issue of Us Weekly magazine: "Jen thought of the cast as her family. She still does, but back then even more so because they were together all the time and making it on the fame ladder together."

Meanwhile, Matthew recently revealed that he dropped out of 'Don't Look Up' after his heart stopped beating for five minutes.

The actor was cast as a Republican journalist in the Adam McKay-directed movie, but he was forced to quit the project after his heart stopped beating during surgery and he required CPR treatment to survive.

In his new memoir, 'Friends, Lovers, and the Big Terrible Thing', Matthew - who was having "some kind of weird medical device in [his] back" - says: "I was given the shot at 11:00am.

"I woke up eleven hours later in a different hospital. Apparently, the propofol had stopped my heart. For five minutes. It wasn’t a heart attack - I didn’t flatline - but nothing had been beating.

"I was told that some beefy Swiss guy really didn’t want the guy from ‘Friends’ dying on his table and did CPR on me for the full five minutes, beating and pounding my chest. If I hadn’t been on ‘Friends', would he have stopped at three minutes? Did ‘Friends’ save my life again?"