Sundowns were beaten 1-0 on Friday night by defending champions Wydad Casablanca, leaving them in third position after Horoya beat AS Togo-Port in the other Group C match.
Sundowns have brought in a few players to replace Tau and Billiat, but it will take some time before they perform as well as their former stars did.
“We lost two important players, Khama Billiat and Percy Tau to the English Premier League. So that creates a little bit of a difference,” Mosimane said after the match.
The former Bafana Bafana coach added: “It’s football, it’s the power of the Euro, we have to sell the players. But I don’t think we are weak.”
Sundowns, who entertain Horoya of Guinea in their final group match later this month, will be back in Absa Premiership action when they host topflight returnees Highlands Park at Loftus Stadium on Wednesday (19h30).
Mosimane's men will afterwards visit Cape Town City in the first leg of the MTN8 semi-finals this weekend.