The Netherlands international is in the process of finalising the formalities of a £24 million ($35.91 million) switch to Old Trafford, having earlier in the northern summer announced he would not be signing a new deal with the Gunners.
Press Association Sport understands Arsenal was prepared to make its key striker, who last season bagged 30 Premier League goals, the highest paid player in the club's history, but reluctantly agreed to sanction his departure when it became clear
It is another blow to Wenger's vision of building a young squad capable of dominating
The Arsenal manager admits
"Robin is an exceptional player in the pure sense of the term. I haven't seen many in my life who have that quality in the first touch, are so technically gifted and have that intelligence of movement in the final 20 metres," said Wenger, during a stint as a media pundit for French TV station TF1.
"Unfortunately, there is an economic reality and the desire of the player. There is no other way but to let him go.
"It is another pill to swallow, we have swallowed other ones and we have always survived, but he scored 30 goals for us in the league last season, practically one every match."
Wenger added: "The economic factor is the sole satisfying thing, but you don't play with money on the pitch, you play with the quality of the players. To lose a player of this ilk is tough."
The northern summer additions
Santos told ESPN: "We lost a lot.
"Today we have experienced players and also the arrival of some new players, like
"We are well focused, the team is well prepared and well motivated
"We have brought in some top quality players and I am sure that Arsenal will have a great campaign in the English league."
English Premier League