Show me where he says that. That's ridiculous. He's already played Sigurdsson in the pivot role he played Cleverly in. He's played him in central midfield in a 4-4-2. Silva's 4-3-3 is played with either 1 or 2 DMs.
Here are his exact quotes:
"One second season I used 4-4-2 and sometimes last season three at the back in some moments but 4-3-3 is my system, depending on my No6."
“Depending the profile of my No6, I can play one and two or I can play two and one behind the striker depending on the profile of our No6," Silva continued.
So...under Watford, his preferred 4-3-3 was in essence a 4-2-1-3. Capoue / Doucoure at No 6s, Cleverly as a pivot 10. Really, those twin 6s play more box to box and his 10 usually sat deeper with the wings and CF pushing up the pitch.
When he says behind the striker, he's talking about a position that Sigurdsson can play. With us, he's playing a 4-3-3, that is really a No. 6, an 8 in Gomes, and a 10 in Siggy. I actually think he'd like a Scheniderlin like player to be like Willam Carvalho - and Wilmar Barrios, so he can play both, but he doesn't have the players.