Ronaldo, 37, made his sensational return to Old Trafford last summer and scored 14 times in 24 appearances in his second spell for United.
There are reports that Ronaldo could request his exit from the Red Devils if United fail to secure Champions League football next season.
The 22-year-old Swedish striker was in superb form last term and scored 17 times in 34 La Liga appearances for Sociedad.
However, Isak’s goalscoring form has plummeted in the current campaign and the Sociedad star has netted four times in 18 Spanish league appearances.
The Daily Mirror reports that United and Chelsea are “both passionate” about Arsenal target Isak.
Arsenal reportedly saw a £58m bid on deadline day rejected by Sociedad, with the La Liga club wanting the Gunners to meet Isak’s £75m release clause.
United have now identified the former Dortmund striker as a ‘potential Cristiano Ronaldo replacement should he leave this summer’.
However, the Red Devils will face opposition from Chelsea ‘on the attacking front’ if Romelu Lukaku is unable to regain his goalscoring form.
Arsenal will ‘also be in contention for Isak’ as Mikel Arteta aims to sign a new striker, with Eddie Nketiah and Alexandre Lacazette likely to leave this summer.
Isak is ‘considered an alternative’ to Borussia Dortmund striker Erling Haaland, who is in high demand by top European clubs.
Despite being contracted until 2026, Isak insisted he was focused on his career at Sociedad and not his future elsewhere.
“I’m a Real player, I work here and it’s my club. My job is on the pitch, and as I told you before, there are other people working outside.
Asked about leaving Sociedad, Isak added: “You can never be sure of anything.
“As I always say, you never know what can happen in football but in the end I’m here, happy, and nothing more.”