'Sanchez the right fit for United'
Sanchez on Monday sealed a move to United from Arsenal in a swap deal with Henrikh Mkhitaryan, saying he was "thrilled to be joining the biggest club in the world".
The 29-year-old Chile striker, who came close to joining Manchester City last summer, has signed what is believed to be a four-and-a-half-year contract that will make him the highest-paid player in the Premier League.
It has been widely reported that Sanchez will earn a pre-tax salary of £500 000 ($697 000, 569 000 euros) a week at Old Trafford.
"Manchester United signing Alexis Sanchez is similar to Robin van Persie in the sense that they're getting a ready-made superstar and you're seeing the contribution already with Anthony Martial upping his performances," new Wales manager Giggs said on skysports.com.
"That can happen when top players come in, those around them lift their levels, and there will be some difficult selection decisions for Jose Mourinho.
"However, Sanchez can also play on the right, as he did at Barcelona, so if Martial is flying then he may have to play in a different position, but those sort of decisions are positive for a manager to have to make."
United boss Mourinho said Sanchez would "complete our very young and talented group of attacking players" alongside the likes of Romelu Lukaku, Marcus Rashford, in-form Jesse Lingard and Martial, who has scored three goals in his past three Premier League games.
Giggs, United's most-decorated player, said it was an extra bonus that the former Arsenal man would be able to play in the Champions League – United play Sevilla in the first leg of their last-16 tie on February 21.
"To have somebody like that, who won't be fazed by playing in the big games, makes the squad look really strong," he wrote.
United, who have not won the Premier League title since 2013, are second in the table but trail Manchester City by 12 points – a gap that looks impossible to breach even with four months of the season to run.
Sanchez took a parting shot in an Instagram post at ex-Arsenal players who have criticised his attitude.
"There are people (former club players) who have spoken with no knowledge of what happens inside the club and cause damage," Sanchez wrote.
"I must say I always gave 100%, until the last day, when I asked to the Mister (Arsenal manager Arsene Wenger) to be in the team, because I wanted to be a contribution."
Armenia international winger Mkhitaryan, also 29, fell out of favour with United manager Mourinho after just 18 months at Old Trafford.
The Sanchez-Mkhitaryan deal is the latest move of a busy January transfer window during which Liverpool sold Philippe Coutinho to Barcelona for 160 million euros and signed central defender Virgil Van Dijk from Southampton for £75 million.
Reports in Germany say striker Pierre-Emerick Aubameyang could be on his way to Arsenal from Borussia Dortmund.