Lukaku and Matic were signed for a combined £115m from Everton and Chelsea, respectively.
Ball thinks Toffees should have made an offer for Giroud while he was on holiday.
Ibrahimovic was re-signed by United after he was released earlier in the summer.
Ibrahimovic has been sidelined with a knee injury sustained at the end of last season.
United re-signed Ibrahimovic, who will return to full fitness in January.
Rooney also gave his thoughts on Jose Mourinho and Louis van Gaal.
Lukaku has scored three goals in three league games so far.
Ibrahimovic was re-signed by United after he was released earlier in the summer.
Premier League clubs spent a combined £1.4bn on players during this summer's transfer window.
United re-signed Ibrahimovic after his contract expired earlier in the summer.
Stuart Pearce believes last season's top scorer would have a role to play for Jose Mourinho.
Lukaku scored in Manchester United's 4-0 win over Swansea City at the Liberty Stadium on 19 August.
The Spaniard's former bosses tell IBTimes UK why the striker will light up Stamford Bridge.
Ince does not think his former side can totally rely on new signing Romelu Lukaku.
Romelu Lukaku scored a brace on his first appearance for Manchester United.
Koeman rubbished reports suggesting a deal for Sigurdsson was in jeopardy.
Lukaku joined United from Everton in a £75m deal in July.
Mourinho's must perform better in the league this time around after winning two trophies last term.
United looking more like a 'Mourinho team' after transfer business, the former England defender says.
Lukaku will make his debut for the club on Tuesday (8 August) in the European Super Cup.
Lukaku joined United from Everton in a deal that could break Red Devils' transfer record.
Matic has reportedly completed his United medical on Sunday.