However, he is currently serving a 13-week ban for biting, his second suspension of the year.
"If you're asking me if it's the right move from a rugby point of view, to develop his game or develop him as a person, I think it's not and I told him that," McCall was quoted as saying by British media on Wednesday. "But he obviously doesn't think that."
Ashton on Tuesday explained why he was leaving, saying: "This was easily the hardest decision I have had to make... It will be a real wrench to leave. I feel the experience and opportunity to play in France was too good to turn down."
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He has been crucial to Saracens' success since joining at the start of the 2012-13 season and McCall believes he would continue to develop if he stayed at the club.
"His game has come on leaps and bounds at this club. This is the right club for him to grow as a player and as a person. It's unfortunate that he's decided what he's decided," said McCall.
Ashton switched from rugby league, where he played for Wigan, to join union side Northampton in 2007.
He made his England debut in 2010, scoring 19 tries in 39 tests, but has not played for his country this year.
Ashton's move to France's Top 14 means he will be ineligible for England under the rule preventing players outside the Premiership being selected for the national team.
"The international door, in his own eyes, seems to be shut. He's almost 30 and this is probably a big opportunity to get a big contract," McCall added. "I do understand his reasons for going and it's disappointing from our point of view."
(Reporting by Nivedita Shankar in Bengaluru; editing by Ken Ferris)
