The Celtics regular starting lineup is all players with single-digit numbers - the only 5 single digit numbers that are not retired. I doubt that has happened since before Bill Russell's day. (Russell played with Cousy #14, Heinsohn #15, Sharman #21, Sam Jones #24, KC Jones #25, Loscutoff #18, Havlicek #17, Ramsey #23, so he likely never started with 4 other single digit guys). Maybe you need to go back to the 1940s to find it.
You're probably right about a regular starting five.
Early 2000s were close to a starting five for a couple games - Walter McCarty (0), Tony Battie (4), Kedrick Brown (5), Kenny Anderson (7), Tony Delk (7 in 2003), Antoine Walker (8), Milt Palacio (9), J.R. Bremer (9, 2003)
Looking at the box scores for 2002 - 2003..
- a lineup of of Battie, Delk, McCarty, Pierce and Walker started on Dec. 9, 2002 (4 out of 5)
- another lineup of Battie, Bremer, Brown, Walker and Eric Williams started on March 7, 2003.. (4 out of 5)
One of Eric Williams or Pierce started ever game.. so it never happened with all 5... and obviously they were the regular starters.
On the Celts it's interesting how many numbers have been retired... combinations become more difficult... even in the single digits you have Walter Brown (1), Red (2), DJ, Garnett and Russell... so you really only have 0, 4, 7, 8, 9 after 2013 (as you said). As it is.. 00, 0, 3, 4, 5, 6, 7, 8, 9 are the only numbers that were ever used... and some of those numbers weren't used for decades.