That's "not really enough to excite anyone," Weil told CNBC's "Squawk Box" in an interview. "But this is a long story. We never thought this was a six-month story. We're looking forward to years together to build a substantial business."
Nicknamed on Wall Street as the bond king, Gross announced his departure from Pimco, the investment giant he founded, in September 2014 to join Janus Capital.
As for the performance of Gross since getting up and running at Janus, "Bill went through some volatility," said Weil, who himself spent 15 years at Pimco before joining Janus in 2010. "I think it's a little bit silly to get too focused on too small a performance time period," he added.
"But more recently his performance has been strong," Weil said—hopeful that a bet by Gross against the German 10-year bund would pay off.
Read More Bund buyers beware: Bill Gross is coming
Gross referred to the trade as "the short of a lifetime" in a tweet.
"I think [Bill Gross] is well-positioned," said Weil. "In a way, his performance has reverted to his longtime standard [of] excellence. He's far ahead of his competition in the six or seven months" since he's been on board.
Bill Gross declined comment.
Last year, Gross put $700 million of his own money into the fund.