How is it not plausible? United's midfield could use upgrading. Therefore, they try to upgrade with Thiago and/or Fabregas. Ronaldo is one of the best players in the world. Therefore, they try to acquire him. Both of those things are plausible.
Your grasp of the English language is worrying.
Both separately are plausible, but both happening is not plausible.
You're confusing what's "hypothetically possible" with what's "plausible". It's hypothetically possible that Wilfied Zaha, who has shown us incredible stuff in pre-season, could go on to become the greatest of United legends. Possible, but not plausible because we have so little factual basis to go on to warrant that conclusion that it's simply beyond the realm of reasonable belief that reach that conclusion. But possible? Yes, Zaha could go on to smash Charlton's goal scoring record and Giggsy's matches played record and be hailed the greatest United footballer of all time.
The word "plausible", at least in this context, is close to being synonymous with 'believable". To hold that an assertion is plausible, it has to be more than hypothetically or theoretically possible. It's possible that the dude who uses the name "Ruud10" on RedCafe is Stephen A. Smith (an American sports commentator), but it's highly unlikely that that is the case. It's just not plausible, although it could just be true.
Reasonable people may hold differing opinions on just about anything. Was it once viewed as plausible by intelligent people that Aryans were the "master race"? Yes, it was. Was it once widely viewed by learned men and women that phrenology was plausible? Yes, it was. Both theories were debunked as pseudoscience, while other perfectly respectable theories, such as the sun spinning around the earth, have been debunked by improved fact-finding technologies such as telescopes.
You, a reasonable poster (although I question your choice of username), believe it's entirely possible, and therefore plausible, that we could land BOTH Fabregas AND Ronaldo -- or at least that someone who holds the keys to Old Trafford thinks we could land both. I agree that it is possible -- meaning nothing more than a logical possibility -- but I also believe it's extremely unlikely to the point of self-evident absurdity. As we sit here right now, it seems highly unlikely we'd land EITHER, let alone BOTH. But there are those, and perhaps you are one, who think it quite plausible that we'd be genuinely in for both. (No smokescreen.) Few things that could transpire over the next 30 days that would please me more than the historic spectacle of both Ronaldo and Fabregas holding up red shirts with black, button-down collars at Old Trafford, proving that you were right -- that it was indeed "plausible" because it actually came to pass -- and proving me wrong. I don't consider myself one of the dour posters here, but even the most optimistic of caftards would reject as wildly unlikely the prospect of seeing both Ronaldo and Fabregas brought in. But you never know. Until it doesn't happen, as you see it, it could happen, and therefore it's "plausible".
So, if you wish to use the word "plausible" to be synonymous with "impossible", please go ahead. But the usage of that word in that way would be alien to the ear of most English speakers. Or maybe English-speaking Americans. Do Brits who believe something that is highly unlikely but it is theoretically possible, is still "plausible"? Maybe we have a mere American-English v British-English divergence in play.
(My apologies to caftards...when a point of personal privilege is raised it's not in my nature to respond meekly.)